Go down through a hole.
Walk straight ahead. You'll get to the temporary camp of the rebels and vorts. Pass the rebels and get to the table where vort is trying to heal Alyx. There are some goods here and there. Collect them. You will find a sub machine gun.
Soon after you will be asked for help. Rebels will explain to you that every entrance has motion detectors. If any light goes on, the antlions are coming. The more the lights the more the antlions to come.
There are two turrets set to guard the entrances. The thing is, two is not enough. You will have to help the rebels to repulse the attack of the antlions. The first wave is easy. You will just have to put the turret where the light goes on.
Then there will be another wave. This time 2 lights will light up. Sometimes on the opposite directions. However, this won't be so bad. You just have to be fast. It's good to put 2 turrets at the same entrance. If the turrets are down just use your weapons to kill the antlions.
At some point the lights will light up at every entrance. If you still have any turret left use it. You should guard the very right entrance. That's the entrance you see on the screenshot. Anytime the antlion shows up in the range of gravity gun, use LMB to smash it against the ceiling .
Then there will be another wave, this time very strong. You will have to stand next to the table where vort is healing Alyx. Thanks to it you will be able to protect them. At this point you're free to use gravity gun, turret or any other weapon.
When this wave is over, 3 lights will light up at one of the entrances. However, it will turn out that it's not antlions but.. Vorts.
Suddenly 3 lights will go on at every entrance. This time this won't be Vorts but antlions attacking in great numbers. Stand next to Alyx and try to kill every antlion that will approach her. If you're out of ammo, use the gravity gun. Vorts will be very helpful in this fight.
When the attack is over, vorts will try to heal Alyx together. It will turn out, that Alyx's injuries are so grave that it's necessary to get the larvae extract from antlions' nest.
One of the vorts will go with you. When everything's talked over, follow him. He will open the gate leading to antlions' nest. Go inside.
Go straight ahead killing antlions and its acid versions. You will get to the place with many paths.
Still, go straight. Turn left, and then right. Be prepared for heavy attacks. At this stage of the game they come from every direction, even from behind you. You have to get to the green path which is going up.
Go straight, the tunnel will lead you. You will get to some illuminated rocks . Go further until you see corpse behind the web. Destroy it and collect the goods.
Carry on with your walking. Remember to kill the larvas as they contain the essence that can heal you. You will get to a hole. Jump in.