Once you've turned off the electricity in the flooded room, break the vent next to the switch and proceed along it (be careful as there is a fan spinning above you when you exit, that will mean instant death if you're not careful)
Proceed along the ventilation shaft and it will suddenly crumble and drop you on the other side of the room with the locked double doors (the one you cant get through earlier) Move the two metal boxes next to each other and use that to climb up the vent at the top. (You have to look up near the big metal box)
Climb up and follow the vent and be careful before you step out, as you can watch a scientist get riddled with bullets. Quickly jump out, use the boxes for cover, and then go under the space below the sentry gun turret and flip the switch to deactivate it.
Go up the stairs that the scientist came running down from (dealing with a few headcrabs, a couple of Vortigaunts and a sentry gun turret hidden behind some glass near the top of the stairs)
Once you're up the stairs, you will come to a room with a boarded up door. Break the blocks with your crowbar and proceed to the giant meat freezer. You will see another scientist running away from some bullsquids and headcrabs. Deal with them.
Making your way through the freezer, you will notice a big red switch near a bullsquid at the right, which will activate the overhead crane with two crates on it.
Activate it and make your way back to the start of the freezer. Use the small ladder here and climb up and make your way along the vents to the moving trolley that you just activated.
You can see another vent in front of you from here. Time it right and use the trolley and the suit charges on it and make your way to the vent at the opposite.
Proceed along the vent until you come to another set of stairs where you will meet a security guard taking out some zombies. Proceed along the stairs and you will come to an area where a zombie will throw itself from a glass enclosure above you and another zombie will break out of the wall. Show them the mighty power of the crowbar. In case, you want, you can ask the security guard to accompany you.
Once you're done, you will notice an elevator shaft at the end with a ladder at the opposite end. Run as fast as you can and grab the ladder and then proceed to make your way up, to the scientist hanging from the ladder near the broken elevator.
Use the ladder and get on top of the elevator, switch on your flashlight, and then break the grill on top of the elevator and drop in. Press the button on the elevator control panel to enter the next level - "We've got hostiles!"
