dynamicbob
Banned
It could be a thick layer of ejecta covering a structure. We can't exactly get out and dig, can we? It seems silly for an advanced spacefaring society to resort to earthen mounds. The dirt could also be an impromptu layer of insulation and radiation shielding. If you want to build something on a planet bathed in radiation, the easiest way to protect one's self is to build underground.
remember it was biological warfare that killed them off on the end, does this fitin with youre thesis ?