After teleporting for the blue skull key, go through the first skull door on the right. A platform up the stairs with a soul sphere has been lowered. Take this teleporter to telefrag the mancubus and pick up two boxes of rockets (although they can be accessed from outside this secret by walking up to the ledge from its sides).
Amongst possible names for this map during development were Rocky Yard and Emerald Eve.
In the initial release of Plutonia 2, the DEHACKED patch incorrectly names this map Emerald Eve, but the map's title graphic (the lump named CWILV02) correctly calls it Skull Island. An upcoming re-release of the WAD will correct this issue.