The Mountain was a seemingly normal terrain albeit one that was formed by the accelerant, rather than natural causes, it is later revealed to be a living organism that has over-taken the Prometheus wreckage similar to how moss and fungus grow over objects.

RCO185 1469289942

Angela Foster and her crew venture into the mountain and discover a river of acid, they soon realize they are in a "vein" of a living creature. The mountain begins to close on the crew but Elden saves them by pushing it back and consequently becoming merged with it. It can be inferred that the mountain is actually the Deacon creature which erupted from the Last Engineer shortly before. This is supported by the fact the the shape of the mountain very closely resembles the concept art for the Deacon, and that it was in very close proximity to the escape pod in which it was born.