As you enter go left. To the right of the blue door is a lava pit. Jump to the other side to get the Quad damage, then down in the pit - a narrow ledge will prevent you fall in the lava. A lift will bring you up again.

On the right side, as you enter this lava room, is a small skull on the wall. Shoot or axe it to get a goodie. And some trouble.

Shoot at this spot, a secret door opens. Jump down there. Beware of the locals.

Same room as secret #2, a bit lower down. Shoot this bit of wall, and a slipgate appears. Pretty straightforward.