Level 04: What About Bob?
After destroying the second set of wires the lava will begin to rise. Instead of running up the stairs drop of the ledge you're currently on. As you can see from the map there's a secret door that has opened up under the lava. This door leads to a platform activated by a switch which will take you out of the lava and to a pile of ammo. Beware, you'll need full shields to make this journey.