Elaborate Underground Base: The Imperial Military Service (various, with special props going to the Imperial Navy’s hollow hangar moon at Palaxias IIb), the Imperial Emergency Management Authority’s Crisis Citadels, more than a few data havens, the original location of Argyran Depository, the Nightfall Complexes (retreats for city populations in the event of nuclear war, asteroid impact, etc.), and oh, yes, all the entire cities built down there by people who just found that they liked it…