Area 51

Area 51 is a Doom II replacement mod of two levels, created by Keith Hickman in 1997 with graphics and sprite contributions by Garth Donovan.

Custom monsters
Area 51 includes graphics replacements which, among other changes, turn Wolfenstein SS into Alien Sump workers and cacodemons into security droids.

Trivia
The WAD includes the name lump for a possibly planned third level, called The Mothership.