This level is somewhat oddly-named: as you explore, you'll see there's no chasm to be found, but there's an interesting layout with some surprising areas you'll come onto. You begin in a smallish water-filled hall with cages along the sides. If you want, just ignore the Ophidians inside: any items they drop will be out of reach in the cages, and there won't be any reason to come back here after you leave. Go past the hall and through the door upstairs.
Series of rooms
In the next room there's a long window looking out into a big indoor pool, in the middle of which is a structure that stairs lead up to. You'll see, through windows, a couple Iron Liches inside the structure. You can get them out of the way now and save trouble when you eventually do go there, but be careful if you use your Phoenix Rod to do this: there are enemies wandering around in the pool that will usually congregate just below the window, which will result in your taking backsplash damage. After you're done with that, go up the steps opposite the window and pick up the item at the top. When you do, you'll get an ambush, but there are items inside the compartments that opened.
Continue on to the next room, which has a water-covered floor, and on the other side the route continues. In here is the Yellow Door, which appears to lead out to the big pool. Before continuing, there's a bonus area: go to either step out of the water, then step up to the ledge across the room from the Yellow Door. Activate the big panel, which is a hidden door. Hit the switch in here, and a door to the left will open, with a teleporter inside. This goes to a small cross-shaped room. Hit the switch behind you, and a teleporter in here will be uncovered, which goes to another identical cross-shaped room, where there's a weapon. Hit the switch that was behind you when you teleported here, and take the teleporter that is uncovered to return to the initial room behind the big hidden door. (Take a look at the automap, and you'll see how the two cross-shaped rooms are arranged.)
You don't have the Yellow Key, so continue on to the next room. In here, there's a narrow stairway going up the wall to the left, but the lowest step is out of reach. A doorway leads to a darkened area. First, go to the far end of the room where the barrels are, and collect the various items here. Now, go into the darkened area, find and hit the switch. On your way out, look in the compartment where the Undead-Warrior ambush was: there's a doorway to the right that's easy to overlook, with an item inside.
Pool and hall
The narrow stairway is now reachable. Go up to the ledge at the top, collect the Yellow Key, and return to and open the Yellow Door. This, as you probably expected, leads out to the big pool. You'll see that the door to the structure is the Blue Door. (The pylons, although in plain sight, can be easy to miss because they're a ways away, along the outer end of the stairway.)
At the opposite end of the pool from the Yellow Door, a stairway leads up to a tall room with several high ledges. The Green Door is in here, to the left. Go right, to a short hall with columns along either side. Walk up to the switch in the narrow doorway at the far end, but then back away immediately: you'll avoid a minor ambush from enemies that were waiting on either side. Deal with them from out in the hall where you have more room, then hit the switch. A door off to the side in the hall has opened, and a narrow stairway leads up. Be on guard in here: there's a Maulotaur in an adjacent room, visible through a window, and the trails of fire he shoots along the ground can reach you through the window. Hit the switch up here, then return to the room with the high ledges. The low ledge you may remember being in front of the decorative panel has now lowered, allowing you to now reach the panel, which is a concealed door. Get your weapons ready first: this is where the Maulotaur is. In here you'll find four pedestals, two on either side, and a ledge out of reach to the left with the Green Key. One pedestal on the left is a teleporter: step into this, and you'll go to one of the high ledges in the room outside. Hit the switch here, and go back to the pedestals. Another teleporter has opened, on the right, which goes to the other high ledge outside. (Remember, that from up on these ledges, you can jump down to the lower ledges on either side of the entrance to the room, if you need the items.) Hit the switch up here, and go to the pedestals again, where a third teleporter has opened. This one goes to the nearby ledge with the Green Key. Collect it, then, while you're up here, look to your left where the texture on the wall is misaligned. This is a hidden door to a room with lots of items. Before continuing, go to the pedestals again, where there's one more bonus area nearby. If you step onto the fourth pedestal (the one that doesn't have the teleporter sparkles), a compartment nearby will open, with some enemies and a weapon.
Catacombs and Outdoors
Now that you finally have the Green Key, open the Green Door and go down the long stairway, where you'll come to a network of catacombs and an underground pool. Thoroughly explore this area. You'll see that the catacombs encircle most of the pool. Eventually you'll come to a passage that goes through a small pool and then continues up a winding stairway. Just before the pool there's a small section of the passage that's lit by a torch. Activate the flickering section of wall opposite the torch: this is a hidden door to a room with a weapon.
At the top of the winding stairs you'll emerge outdoors, looking over a huge expanse of icky water with a high walkway crossing it. This must be the level's namesake chasm, although this area is much wider than deep, and more like a valley. Be careful here: there's a strong wind blowing from the left, but you'll find out the wind is only above the walkway, and not the water. Fairly close by to the right is a window in the cliff face looking into a room with enemies and a bunch of items. To the left, across the water, is another window to a similar room. (Tip: use your #4 weapon to fight the Gargoyles. Weapons like the Crossbow will be blown off course by the wind.) On each side of the walkway, down in the water, you'll find a teleporter. These, of course, are your way out if you fall in. Don't forget the Shadowsphere on the nearer one. Run over the walkway to the other side, and collect the Blue Key in the small cave on the other side. Get the Wings of Wrath too: although you're "officially" done here, you can now reach those rooms with the items. Fly to either end of the valley: on either end is a route leading to that side's room. There are Wings of Wrath in each room too, although if you go quickly you can do both sides while your initial set is still active, and have at least one extra set to use elsewhere in the level. As you leave the "upwind" room, a side area along the path outside will open, where there are items.
After you're done here, return to the catacombs. You'll run across a few more enemies that weren't here earlier; they were waiting in compartments that were closed and hidden. Be sure to look for the compartments, because each has a good item. There are four in all; one has a Map Scroll that'll make it easier to find any you missed.
Structure at pool
Once you've explored everywhere you want, return to the big pool, and open the Blue Door to the structure. In here you'll find a door with the exit symbol, inside which is the exit switch. This was the final full-size level in this episode; up next is the finale. Although there is much ammo there, it doesn't hurt to gather all that is available here. (If you're seeing that you're missing a few kills on the end-of-level tally, remember that they're just the Ophidians in the cages at the beginning of the level, if you passed them by.)