"Aye...and the wealth that our forefathers hid beneath the massy entrails of the Earth"
SOME THOUGHTS ON THE MATTER
Underground facilities are architecturally focused on the interior dimension, underground structures only have an interior. they are without an exterior perspective - unlike surface structures which are architecturally focused with an exterior in mind.
Underground facilities are self-contained structures that have little or no primary relationship with other facilities.
Underground structures can exploit the lack of urban constraints as that with the case of surface dwellings. With no streets, roads, highways, commercial, industrial, residential or rural zones, underground buildings can assume a shape most practical to the purpose of the facility. Many descriptions imply such facilities take on a round or cylindrical profile - as befitting of a deeply buried structure.
Underground structures or sub-cities are physically separate entities - separated from the socio/cultural/political/economic forces of the surface world with its nations, economies, political relationships and social dynamics. It could be said that they are worlds unto themselves.
Elements in science fiction have at various times dealt with the idea of underground bases or sub-cities, eg: THX 1138, The Andromeda Strain, UFO television series.
Preparing your minds for a new scale of things...
From the 1956 film Forbidden Planet, the Krell underground station is Hollywood's most impressive depiction of an immensely large underground structure. Self-running and maintaining, the base is a testament to a highly advanced culture.