Rocktopi
The Rocktopi are the aliens that were first introduced in Earthcore. They have a spherical body, with six limbs, that pulse different colors.
Story
Once the Rocktopi were an advanced, star-travelling race living on their own world. Their planet was destroyed by a rival, wasp-like alien race. At least one ship survived, and the species was perpetually pursued by their genocidal enemy. To escape from certain doom, the Rocktopi searched for a planet to live on. They found Earth, and decided to hide themselves there. They chose a mountain, in the middle of Utah, which would be nicknamed "Funeral Mountain" in the future. They cut a square hole, 1 mile wide by 1 mile long, and 3 miles deep. They lifted up the mountain, and put their ship under it. They considered it much better to die, than to be found by their enemy. In case they should ever be found by their enemy, they created a doomsday device. It was a bomb to go off miles underground, collapsing fragile supports, and turning the mountain into a volcano. They also chose another mountain, Mt. Fitzroy. It is assumed that they did the same things there.
The Plague
The Rocktopi had a plague which wiped out over 70% of their population, killing much of their knowledge and causing problems with interbreeding. They then had to rely more and more on the silverbugs and regressed into primitive creatures wholly controlled by the silverbugs.
The silverbugs (though quite possibly the Rocktopi also) are extremely violent, killing everything that came near the mountain. Before long, animals, and Native Americans knew not to go near the mountain.
