The Mountain is a level in Terror from Outer Space. Completing this level allows access to the Volcano. The creatures you will encounter are Adult Bleeches, Troggs, Cave Troggs, Rockadillos, and Tortusks. There is also a Vorticon Car hidden in this level. Keen can use this car to take him to the Secret Cloning Lab, the secret level.
There is a sign on the top of the mountain, which reads (in Standard Galactic Alphabet): These are not the caves of oblivion. This is a reference to the Keen 3 level "Caves of Oblivion" in which Keen can abbreviate the level by bypassing the mountain.