Swim through this hallway and it will eventually rise out of the water. It will fall over and the door will open. Grab the ammo scattered here (don't miss the crossbow bolts, which you will need later), then push the computer bank blocking the metal door out of the way. Follow the tunnel, taking a right at the fork, and swim up the vertical shaft. Stop to get some air, then break the grate on the floor and swim through the tunnel. Soon, you'll get to a hallway with a blocked metal door. When you reach the pile of debris that you must duck under, you're almost there. Though you can make it all the way through, there is a metal door about halfway through that leads to a small room where you can take a breath of air, but you will be continuously bitten by hagworms. Keep moving through the passageway, stopping for nothing, lest you die of asphyxiation. Look for a passageway near the bottom and head through it, swinging the crowbar wildly at any hagworms that get in your way. Equip your crowbar and dive down to the bottom. Jump down to the side you haven't explored yet (the side without the barrels).

There are two sides divided by the concrete barrier. Continue up the stairs and out to the ledge. There's some ammo in crates around here as well as a battery. Through the red doorway, you'll run into two mawmen at the stairs. Jump onto the side that doesn't have floating barrels.

Half life 1 chapters