So the first thing you need to do is get to the Lab. The Lab is one of the maps you can find while navigating.
I recommend you bring an O2 tank into the Labs.
It is very likely you drown if you don't.
A blow torch is an optional item you can bring.

The labs are a very large map, but it can be easily navigated if you know what you are doing.
When you enter the lab your goal should be to swim down.
The Room you are looking for always spawns at the bottom of the map.
If you ever end up in the medical test center with The monster cages go back to the entrance and take a different way down.
You are looking for this room.
This room has a big tube in the middle with the computer room off to the side.
When you find yourself here you know you are going the right way.
Continue until you find a dark shaft heading down.
At the bottom should be this minigun room.
Continue into the connecting Globe room.

You are in the room with the globe in the center.
Continue into one of the two connecting rooms, do not go back the way you came.
You want to enter the room that looks like this.Swim to the back and you will see a vent with a Rat in it.
Swim through the Rat or use a torch to break through the wall instead.
You will end up in a room with a desk on this desk is your code, write it down.
Go back to the Globe Room.
There should be another path other than the one you used to get down here branching off the Globe room.
Go down this path.
You should end up in a room that looks like this.
Blowtorch your way through the wall or go through the vent on the ceiling.
You are now in the detonation room. there is a terminal for you to enter your code.
It works the same as the vending machine.
Once you enter your code you will need to escape the facility before it explodes, remember you are in the deepest part.
Remember the route you used to get in.
Do not leave on your submarine before the lab explodes.
Good luck!
Have Fun!