After doing so, jump down on the platform. Take the pistol (A) and head left.
You'll get to a hole leading to the lower level. Jump down.
Try to jump on the planks and kill the antlion. Then get the goods from a locker.
Then go back to where you landed and head right.
You'll get to another hole. Jump in. It's pretty deep so you'll have to jump from one joist to another (A). You'll get to the entrance to antlions' cave. Go inside.
When you're in, go straight. To destroy the webs blocking your way use Gravity Gun(LMB). After walking through the shiny tunnel, you'll get to another hole. Jump in and be prepared for a fight. This is the territory of the acid-antlions.
Hiding behind the trollies, kill acid-antlions so you can get to the corridor with a way to the left (A) and antlions behind the barricade (B). Try to get to the A point as fast as you can. If antlions are too much of a trouble, kill them.
There's a shotgun on the table and a locker with some goods. Collect it. There's also a hole blocked with some planks. Smash them and get in.
You'll get to another pretty big tunnel with springs and antlions. First of all get rid of the bugs. When it's done, get to the cave and head right.
You'll find a path which will lead you to the lake. Go straight, exterminating the bugs. You can use the gravity gun to push them into the water as well. You'll get to the end of the path and a hole. Jump in.
You'll get to another cave with a big crystal lake. Use it to drown the bugs. Just make them fall in. Do the same with the bugs that will show up on the right. When it's safe, dive into the lake and get through the underwater tunnel to the right side of it.
There you'll have to deal with antlions and barnacles, which hang at the ceiling and try to get you with their tongue. Use the gravity gun to push antlions towards the barnacles. The rest of them, just kill.
Afterwards kill the barnacles and collect the goods.
Go straight killing the antlions that will stand on your way. You will get to another lake. Then you'll find yourself in illuminated tunnels. You will have to struggle with antlions all the time. Finally you'll get to the place where next chapter begins - "This Vortal Coil".