So we've finally reached the surface. Surface Tension is one of the largest levels in the game, both in length and sheer size. Outdoor environments have properties that allow a number of unique situations and enemies that you have not yet faced. Because some of these situations involve combat in large, complex areas, there's no one specific way to play these levels. You'll have to use your experience and creativity to analyze the particular situation that you are and play it accordingly.
 The Dam
Walk through the garage door before it closes, but be wary. There is a marine to your left and one to your right, as well as several more a little farther waiting to ambush you. Using the magnum or shotgun, take out the ones right outside the entrance. Shooting the explosive barrels may take out a few of the ones farther up the canyon and save you some health and ammo. Grab the ammunition the grunts dropped and proceed with caution. Stop near the end of the tunnel where you are still somewhat protected and survey the next area from relative safety.
Here's the situation: up ahead, the road goes over a dam and ends at a closed blast door and an artillery gun emplacement. If you try to run straight across the dam, artillery fire will blow you to pieces. There is an Apache helicopter circling above that can't see you for now, but will fire at you once you are out in the open. The left side of the dam is dry, but is an unsurvivable drop. The right side, on the other hand, is full of water, but contains an Ichthyosaur (I hope you have some crossbow bolts). Every once in a while, the Apache will fire on the Ichthyosaur damaging it a little more. Technically, if you wait long enough, the Apache will kill the Ichthyosaur, but whether or not you wait for this is up to you and the supplies you have. There is a tower on the right side in the lake that has a protective alcove with a health station and a control that controls the turbine. You will need to hit this button to turn off the turbines. When you have done that, you'll have to jump into the lake and kill the Ichthyosaur. Once the Ichthyosaur is gone, swim back down to the bottom of the lake and find the valve wheel. Turning this opens the grates covering the pipes through the turbines. As long as the turbines are off, you will be able to swim through to the river on the other side of the dam. Grab the battery on the rocks and duck down underwater when the Apache is about to fire at you. Follow the river down the canyon and climb the ladder onto the structure at the end. Crawl into either pipe to escape the Apache.
 In the Sand
Whether or not you killed the Apache helicopter at the dam, there will be another one circling overhead here. The pipe terminates in a deep canyon, giving you two choices. The left path goes to a cave with a few headcrabs, a dead marine, and some supplies. The path to the right leads to several crates of ammo and a ladder in the concrete wall. Climb the ladder and jump across to a small area with some batteries, ammunition, and high walls for protection. This is the best place to take out the Apache. Get out your Tau Cannon and charge up the secondary fire mode when you hear the Apache approaching. As soon as you see it, hit it in either the cockpit or at the base of the rotors. Take shelter behind the tall concrete walls when it shoots at you and repeat this as many times as it takes to bring it down.
With the Apache gone, this part of the level is a whole lot easier. When there is no more danger, jump back to the ledge and follow it around to find another ladder. At the top of this ladder is a rather large military fortification in the sand. Follow the rock wall to the left and take the first passage you come to. This leads to a small sandy area guarded by a large green tentacle. If you need the supplies within the tentacle's reach, crouch to avoid making sound and throw a grenade for distraction if you have to. If you shoot this tentacle a couple of times, it will retract for a short while, allowing you to get the stuff scattered near the tentacle's base.
Go back out the way you came and follow the ledge to find another tunnel in the wall. This leads to an opening containing three grunts. There is a large boulder on the right side of the clearing and a tower on the far side. The crossbow or magnum are good for this long range battle. When the marines are gone, head into the tower and turn the wheel marked "Storm Drain Hatch". There is also a small alcove in this area with supplies, but it is booby-trapped with a laser mine. The other alcove in this area leads to another opening filled not with marines, but with landmines.
Save constantly from here to the drain opening. The landmines are mostly out in the open area, so stay to the right side as you make your way through this area. Crawl under the fallen rock, making sure to stay as close to the rocky wall as possible. There is a mine in the sand right in front of the barbed wire, so jump over it or shoot the area until you hit it. Once you've made it to the slightly darker colored sand slope on the far side of the barbed wire, you're safe, as there are no mines on this slope. Jump over the barbed wire and climb down the ladder into the storm drain. At the bottom of the ladder, smash the grate and crawl through the tunnel.
 The Cliffs
At the end of this twisting concrete pipe, you will see a jet fly by. Before you step out of the pipe, inch forward looking down and to your right. There is a turret on the lowest cliff that will continue to pester you if you don't knock it out now. There's also a marine the ledge above it that you can take care of before you even leave the pipe. When you do step out of the pipe, be careful, as you are not given much room. There is a marine stationed on the ledge below you, so inch up to it and look down. When he's dead, walk over to the left side of the ledge and spot the overhang that you can jump to before landing where the marine was standing.
When you've safely landed on this ledge, you have a couple options;
 Route 1
Creep over to the edge on the left and look down. There are a series of ledges that you can jump to in order to reach the lower level. One of these ledges leads around to a cave containing a few health packs if you need them. At the very bottom of the series of drops is a battery and a rather rickety looking wooden bridge. If you try to walk over the planks, they will snap and you will fall through. Since this hazard suit isn't equipped with a parachute, walk over the bridge's support cables instead. Around the corner is a sole marine hanging out on the ledge. The crossbow or magnum are a good choice here, as enemies are visible at long range and there is hardly any cover. Continue along the ledge past the large concrete structure until you get to a cave.
 Route 2
Creep around the ledge, and take out the marine in the cave any way you see fit. Once he's gone, you'll see a pipe going down to the next level. Before you play on the slide, beware of another marine at the bottom. Crouch by the edge of the cliff and inch out until you can see him. A headshot with the crossbow should take care of him. Take a ride down the slide onto the metal platform. If you go left (when looking away from the cliffs), you'll arrive a the rickety bridge taken on route 1. There's nothing much to see here, unless you wanna get the ammo dropped by the marine you sniped earliler. To the right, just around the corner is a cave.
 The cave
There is a lone marine in this cave guarding a powerful weapon. There are a plentitude of health packs and batteries in this cave as well as a Rocket Launcher. Pick up the RPG, as you will need it shortly.
Upon leaving the cave, an Apache attempts to ambush you. It may have worked if you hadn't just found a handy new weapon. Give the Apache a rocket or two to the cockpit to send it into the canyon. When the Apache is down, switch to the pistol and continue along the ledge to a cable ladder. This will take you to a higher ledge with a convenient ramp in the rock and several ledges you will need to jump to in order to reach another cable ladder. When you've climbed this ladder, you'll notice a pipe sticking out of the cliffs. There's a headcrab waiting for its next victim, so peek around the edge and watch the foolish alien jump to its doom. Jump into the pipe and crawl away from these evil cliffs.
 Open Combat
There are many different ways to do this next section, as each path here leads to a different part of the area, but for the easiest way, take a right at the junction where the pipe splits. This pipe will drop off into a canal filled with ankle-deep water. Wade straight across and enter the pipe on the far end. Take a right when this pipe splits, taking you to the end of the pipe and a ladder. This ladder will take you out to the grunt's fortification behind an armored tank. From this position, the tank can't hit you, so concentrate on taking out the scattered marines in the area before destroying the tank. Make sure to destroy the tank with satchel charges or grenades, saving the RPG rockets for a more dire situation.There is a health station and several crates of supplies, including satchel charges and RPG rockets, in the garage behind the tank. When the area is cleared of enemies, find the guard station next to the large door and press the button inside. This will open the door, granting you access to the next part of the level.
Around this corner, a rocket shooting tank is waiting for you. Take out the RPG and turn off the laser sight. Hide behind one of the non-explosive boxes until the rockets stop flying by. When it's relatively safe, peek out and fire a rocket at the tank's dome before sneaking back into cover. A few rockets and explosions later, the tank will blow its top, allowing you to safely pass. Before you do, however, jump through the hole in the wall to your right to find two marines guarding a small outpost full of supplies. When you have scavenged all that you can from both the small outpost and non-explosive boxes up top, walk past the tank and through the large garage-type door.
At the other side of the door, you'll see the aliens dropping in reinforcements. When the aliens are down, go to the right. Detonate the laser mine around the next corner and proceed to the small flight of stairs. Destroy the laser mines in the path, but don't go too far. A wounded security guard pleads for help, shot by a sniper. From the top of the stairs, look up to see a window with netting over it and a gun sticking out. Lob a grenade into this alcove to take out the sniper (when he's dead, there will be a blood spatter under the window). Run past the dead security guard to reach yet another minefield.
There are land mines on this part as well as around the corner, so two well-placed grenades should clear out all the mines. Beyond the mines is a section of barbed wire you must crawl under followed by an enclosure containing two electrified towers, one leaning onto the building. Shoot the two explosive barrels next to the electric converters, causing them to shut down. Climb the fallen tower onto the building's roof and jump down through a giant hole to enter the building.
 Ordnance Storage Facility
The scientist locked up in this room will tell you not to detonate any mines in the next room, for it's filled with explosives. You would do well heeding his advice, for if any of the mines or explosives are set off in this building, whether by you or by an enemy, the entire building will go up with you inside. You should save often while navigating the next few rooms.
Break all the boxes in the room you fall into for supplies, then head out into the hall. Jump over the first laser mine and either use the box or the railing to get over the second. The room past the metal door to the left is another set of boxes. Do not destroy a box unless you know for sure that there's not a laser mine attached to it (a clip of ammo just isn't worth the risk). For now, ignore the door and continue straight down the hall, jumping over another laser mine. Crawl under the final laser mine into a large room where you see what the scientist was so worried about- a pair of rockets hang from the ceiling just asking to be detonated.
There are two headcrabs wandering around the room, so find them and render them motionless before they jump through a laser mine. Follow the wall, staying well out of the lasers' path, until you get to three or four boxes stacked on top of each other. Don't destroy any of these boxes but, instead, crouch and crawl under the small space created between the two bottom boxes. Once you're through, turn right and spot the box larger than the others, whose face marked in orange and is already broken open a little. Smash this side of the box with your crowbar and it should shatter without disturbing any of the other sides. Walk through and break the other side of the box to get to the other side. From here, go left and find the lift. Break the box here that would otherwise have set off a mine above when you activate it later.
Turn around and follow the wall on your left toward the red light. Go up the stairs here, ducking under the laser mine. There is a single headcrab up here, so proceed slowly and shoot it before it has a chance to jump. The stairs lead to a small control room with a button near the window that calls a lift (make sure the box on the lift is destroyed before you call it). Walk out onto the lift and look out over the room. In the very center, surrounded by blue-green lasers, is a a larger lift surrounded by yellow and black stripes. Though the jump from up here is possible, it is difficult, so save beforehand in case you miss it. If it is proving to be a difficult jump, you can always push one of the boxes on the lower level closer and jump from the ledge to the box to the lift. When you're safe on the lift in the middle of the room, hit the button that can't decide whether to be green or red to descend away from this room of fiery death.
 Aircraft and Tanks
The lift will take you to a storage room with boxes of supplies and a Hivehand. As you leave this room, you will pass a security guard who seems to be running from something. In pursuit are two grunts that you can blow up by shooting the explosive barrels as they run past. Farther down the hall, there's a military truck with a live turret in the back. Destroy the chaingun in the back of the truck and grab the supplies there. You'll find yourself in a large garage-type area with a large ramp leading out. Save the game, then run up the ramp into the clearing.
At the end of the ramp, immediately turn right and go up the ramp on the side and watch as a bunch of alien grunts get dropped, then subsequently bombed by a fighter jet. By the time you've smashed the boxes up here to get batteries and health kits, there will be a tank, two marines, and many explosions and body parts in the road. Peek around the corner to make the marines come to you. When these two marines are dead, it's just you and the tank. Equip either the Tau Cannon or the RPG and peek out behind cover just long enough to take a shot at the tank's turret before running back behind shelter. After the marine has been destroyed, run up to it and go down the alley to its right.
There's another sniper guarding this alley, so make sure to toss a grenade up into his nest before proceeding. At the end of the alley on the right, there's a door that leads to a small room with two grunts. Kill the grunts and climb the flight of stairs. Up here is one of the largest and most diverse ammo drops available in the game (the largest is before the warp to Xen). Find the security guard up here and lead him to the door marked "Storage". When you leave this place, if you're not full of ammo on every weapon, you'll be close to it. Make sure to check every box in every room for supplies.
When you have just about all you can carry, go back to the flight of stairs, but don't go down them. Instead, look for a window and hop out onto the ledge. Follow the ledge around the building and back into the alley you previously were in. There are two structures against the either of the walls. Drop down onto one, then jump to the other. Look for the red ladder here, jump to it, and climb it over the wall.
The ladder brings you to the top of a building whose roof has collapsed. Find the diagonal beam that can safely take you to ground level. There are three marines down here, so equip the assault rifle or shotgun before descending. There are some first aid kits in a hidden alcove as well as some other goodies. When you're done, climb the diagonal beam back up to the roof. To cross this roof, you first need to walk across the thin piece of metal to the small remaining piece of ceiling in the center. From there, look right and spot a ledge that's above floor level. Jump down here and walk through the surviving doorway. Jump to the small island, then to the far end and walk through the doorway.
 Big Fun with Big Guns
This door leads to an open courtyard. Walk out into the courtyard just far enough to allow two alien grunts to spawn. Run back into cover and allow human grunts, dropped from a circling osprey helicopter, to fight them for you. When the dust settles, clear out the survivors (the crossbow works well here), and cross the courtyard before the osprey can drop any more marines. Go through the large doorway and turn right to find yet another alien grunt. Fire a few bullets into his stomach to knock him over, then jump up to the ledge above by using the stack of green crates. There is an artillery gun up here that you can take control of. Do so and fire at the compound's front gate (towards the left before the brick wall). A few shots should shatter it.
Quickly, before the helicopter can drop more grunts, jump down from the gun and through the new exit you just made. Run down this hall and equip the assault rifle, saving after the loading screen, but before you round the corner. Beyond this corner is a stationary machine gun. Shoot the vortigaunts you see and take control of the mounted machine gun. An army of vortigaunts begins to teleport in, so mow them down as they appear. Be sure that the vortigaunts are finished teleporting in before leaving your station. Amidst the pile of vortigaunt bodies, you will find a large, circular alien growth on the ground. Step on it to find out what it does- it's sort of like a trampoline. Use it to jump up to the roof.
Approach the edge and look down just long enough for the alien grunts to see you. Back up and wait for the battle between the marines and aliens to die down, then toss a few snarks in to take care of the survivors. Take the health packs from the pile of boxes, equip the hivehand and blast open the grate in the floor. As you crawl through this vent, a pack of snarks will attack. Back up, firing the hivehand, whose hornets should find their mark in this cramped space. Continue through the grate, smashing a grate in the way. Immediately back up, as two marines heard this noise and will shoot up into the vent. Shoot the two marines and drop into the garage below.
When you've grabbed the batteries and used the health station in the corner, open the metal door in the wall, then back up quickly. An explosion will cause the doorway to be blocked, but will open a larger hole in the wall. There are two lifts in this room: one with a car and one without. Press the button near the lift without a car and get on it. Press the button again to lift you up. Shoot the two grunts outside, then jump through the hole. In this courtyard, there's an artillery cannon- perfect for blowing up the door it sits oh so conveniently in front of. Grab the controls of the cannon and blast the door open. Several alien grunts will round the corner to see what happened, so keep firing until they learn that rounding the corner is a bad idea. When all is clear, head through the new opening.
You'll emerge in a rather large storeroom where marines and aliens are duking it out. Hang back until both sides are considerably weakened before popping back out and shooting the survivors. Follow the catwalk around to the ladder and climb down to the floor. There is a small alcove to the side that contains supplies and ammo on the bodies of the dead grunts. Climb back up the ladder, jump the gap in the catwalk, and go through the door on the other side of the room.
 Security Checkpoint
Not too far down this hallway, an alien grunt will knock down the wall to get to you. Kill it, then walk to around to the corner to see a new, open area. Equip the crossbow and shoot all the aliens you can see from your elevated position before dropping down through a hole in the floor onto a pile of boxes. Explore the area, making sure you didn't miss any enemies. There's a security checkpoint out here that requires a security guard to get through. There is one inside the building, so that's where we're headed. When the area is clear, use the alien trampoline in the middle of the road to jump onto the roof of the building. On the gravel roof, you'll find a grate. Smash it and drop down into the building.
You'll land in a pool of water, so climb out and crawl through the only available vent. Near the door, a grunt will open the door and throw in a satchel charge. If you're quick with the crossbow or the magnum, you can kill him as he throws the charge into the pipe. The charge will still detonate, but he will be dead afterward. Either way, be prepared- this is no ordinary satchel charge. Back up all the way to the end of the vent and dive down into the pool of water you initially landed in when you first entered. Look up to see the explosion burn out above your head. Wouldn't it be nice if your satchel charges were that potent? Anyway, crawl back down the pipe and open the door at the end of the tunnel. There is only one marine here, so exiting should be easy compared to what you have gone through so far this level.
In the next room, grunts are battling a horde of headcrabs. The only thing down there of value, except potentially assault rifle ammo, is a health station. Unless you desperately need it, you can run past them while they are distracted if you choose. At the end of the catwalk, you will see a pipe protruding from the wall and a hatch with a valve. Jump on the pipe and turn the valve, making sure to back up before it opens so you don't get pushed to the floor. The pipe will lead to a staircase where a grunt stands guard. Unless you're on Hard difficulty, a well-placed shotgun blast as you exit the pipe should be enough. There's another grunt downstairs behind a row of computers. This is a good time to use one of those laser mines you've been saving up. If you set the trap, then make him chase you, he'll run right through it without hesitation.
When you're sure the area is clear of enemies, go up the stairs to find the security guard you need. Defend the guard well, for if he dies for any reason, you will lose the game. Lead the guard downstairs and out the door down there. This will lead you back outside to the main area. Two vortigaunts have spawned since you were last here in the courtyard, so double-team them, using the sandbags for protection. Lead the security guard to the guardhouse and allow him to punch in the code to let you through. Head through the metal door into the parking garage.
 Air Strike!
At the end of the twisting concrete tunnel, you run into an old friend. A Gargantua is fighting a couple of marines here, so run while he's distracted. Run right toward the parking spots, then right again toward the yellow stripes. Look for the large opening to your left and run through the long tunnel into the large outdoor area. There's an alien trampoline to the right, so use it to jump over the wall and into a large silo of water. Climb out and find the slanted pipe leading to a taller silo. Walk up the pipe leading to higher ground and listen to the radio.
Play around with the controls to see how they work, but don't drop an air strike on yourself! About this time, if the gargantua followed you, it will be banging on the door, trying to get in. Move the crosshairs to where the gargantua is standing and call for an air strike. It might take a few attacks, depending on how accurate your aim is. When the gargantua is destroyed, there are several other targets you need to attack. The first is the large concrete wall around the moat defending the back left corner of this area. Call an air strike on it and it should come crumbling down. The next target is the cellar door on the other side of the wall in the back left corner. An air strike should open it right up. The last target is the radio tower in the back right corner. Call an air strike on it and watch as it falls over, creating a bridge across the gap.
Run down the silo's ramp and over to the downed tower. Run across the tower and down into the opening created by bombing the cellar door. It seems as though the military are bombing the entire facility as the tunnel you're in rocks and shakes due to constant air strikes above ground. Follow this tunnel back underground, through some twists and turns, into the next level.