This is a secret level accessed from Toxic Dump.
Secret Places (4)
This secret is located in the oval toxic waste pool. If you submerge, you can see that it's blocked by a wall. If you continue through the level and deactivate forcefield 01, the wall blocking this secret is removed.
It's pretty hard to miss the fan. Use a jetpack to fly up to the secret inside.
There's some pigcops hiding out in the room with the red access card. If you press against the wall to the right of the computers, you'll find a hidden door.
This is the last room before the acid river. You can open the computer monitors and find some atomic health behind them.