This is a secret level accessible 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.

You should notice this fan on your way to unlocking the blue access switch. Use a jetpack to fly up to the secret inside.

In the room with the red access card, press against the wall to the right of the computers to find a hidden door.

This is in the last room before the acid river. You can open the computer monitors to find some atomic health.

Secret Combinations

These combination switches are found at the top of the long twisting walkway in the cylindrical room. The correct combination opens the red door to the central tower.