You are on a small platform that will eventually break and throw you out into the world. It looks as if there is some sort of invasion. This section is a lot easier than it looks. First run behind one of the rising columns. Take out the vortigaunts and Xen Masters with the crossbow. Then make a run for the other side of the level and into the cave. There is one more vortigaunt inside. You will also see a healing pool and some supplies.
Jump down the hole where the vortigaunt was camping out and head on through the tunnel. You will run into a web. Break open the web and kill the two headcrabs waiting on the other side. Run through to the other side avoiding the spikes.
In the next room, you will find some barnacles and the bullsquid. You will also find some more supplies and a ton of health packs. After running around the room, head up to the webbing to the left of where you came in. Knock down the webbing and the webbing inside the next room. A column will come come down and allow you to get on.
Once on, take out the newly spawned Xen Masters and vortigaunts. Next jump from one platform to the next until you get down to about the level of the flying manta ray ships. Before you hop on to them, look around for a floating rock at the same elevation level. On it, you'll find a an ammo canister. Pick it up if you need it, then jump on the purple manta ray and ride it to the teleporter.
In the next section, run out of the cave and take out the grunts and vortigaunts. They are easily done with the crossbow without much hassle. Keep running around the left of the hill and you will find a healing spring. Back on the other side is a set of alien oil rigs. Take out the vortigaunts and jump down. There are 2 caves. On the left of the rigs are 3 more vortigaunts and precious supplies. After gathering what you need, continue around and jump on the spinning platform, and from there on the horizontal spike. Run back to where you came in and go on into the next cave.
Get your heavy stuff ready as you run through the cave you will come face to face with another Gargantua. Kite him to the entrance of the cave and he is easy to take out. Go back through the cave. You will find some supplies in a purple section on the right. Then head back and jump from rock to rock to the edge of a cliff. Taking out the barnacle will get you some unwanted attention from some tentacles. Using grenades, make your way to the left and onto the rock bridge on the other side. Follow this path to a new cave, and a few grunts. Take them out and grab a few supplies. Then step into the teleporter.
After porting, you will find an open area with 4-5 vortigaunts. They are passive until attacked, and attacking them will spawn in some xen masters high up in the room which would otherwise not spawn. After taking them out (if you chose), start going up. There are 3 levels. The first level has a health kit. But the second has a healing tube. The third level has a conveyor belt with strange barrels. To get up here, take the elevator to the second level and jump off. The elevator will go up to the third level, but not far enough, and then drop back down to the first. When it returns to the first level, jump onto the top of the elevator and let it take you up. Jump to the first belt. When you get to the beam in the middle, jump over to the second belt. Then let it drop you to the next section.
The place will be crawling with Grunts, vortigaunts, and xen masters, but otherwise its fairly linear. When you see pineapple looking barrels, be careful as they hide grunts. Make your way around until you get to a room with a healing tube, supplies, and Red Ventilation shafts.
Get what you need, and crawl in the shaft. Next you will need to drop down. Beams will let you get down safely. After you get to this point, get your revolver out and save as this next section will be difficult. You will follow the vent shaft until it drops you in the middle of another large room. Start shooting at the vortigaunts and xen masters. I found it easier to start with the vortiguants as they do considerable damage, there are more of them, easier targets, and one shot with the revolver will kill them. After you get the situation under control, pull out your crossbow and shoot anything left that you can kill. To get a better angle, you can go right. But eventually you will need to go left. Head up the ramp and into a small crawlspace. Kill the vortigaunts and head to the healing tube. Go back out and start working your way up by riding on the gear.
At this point you will get ambushed by a vortiguant and four xen masters. After taking them out (and going back to heal if need be) you will notice a couple of red doors. Crawling through them will take you to another room with a circular walkway going up. Each of these will take you to a different section but you will find a ton of supplies, a vortigaunt and a Grunt Barrel. If you can kill the vortiguant quick enough, plant a trip mine on the post right next to the grunt. Step away and pop the barrel. At the top of the platform are 2 grunt barrels. You can get to the bottom one by jumping down, or take the safe way and going back through the other red door.
Once you are finished here, go back to the previous room and take the green gear up. Up top you will find 3 xen masters. Run into the room and you will find a healing tube.
Go up one more gear and you will find a strange looking gear with a portal. Jump on through to head to the next section. This will take you to a black room with floating rocks and a red portal in the middle. Jump from rock to rock to the portal and jump in.