Underground hangars are an extremely common feature on both the Cradle Worlds and the Colonies. Their primary purpose is to protect and store military assets beneath hundreds of feet of solid rock in much the same way as the concealed submarine bases of the 20th Century. In the past, these instillation were built primarily to preserve the beauty of the Cradle Worlds from defilement by unsightly military structures.
In the war-torn 27th Century, they are built by the UCM for their excellent defensive capabilities and to protect their personnel from harsh surface environments.
These instillations usually feature large mechanical apertures which open these cavernous spaces to the surface, allowing aerial assets to strike forth or reach safety. Although these are large structures in themselves, they are merely a visible clue to the vast complex of hangars, storage facilities and living quarters lurking beneath the surface.
The gateways to the subterranean world have often been the focus for bloody battles during the Reconquest, as they can be choke points, supply gateways, entrances and escape routes to both enemy and friendly forces, depending on the mission and who has control of these crucial structures.