submerged building
A structure whose usable floor space is entirely underground or underwater.
A submerged building is a structure whose floor levels are all below the surface of the earth or below the surface of a body of water. If the building includes any above-ground enclosure large enough and high enough for human habitation, other than a simple entrance pavilion, then it must be classified as a regular above-ground building, even if the majority of its space is below ground.
Submerged buildings may be above the surrounding grade on one or more sides, as long as they extend below a rise in contour of the terrain and are mainly covered by natural terrain features such as soil and vegetation. If the covering terrain is not connected with the surrounding grade at some point, then the structure is merely an above-ground building with a green roof. If the building slopes down to form a continuum with the surrounding terrain but its roof is paved or constructed, then it is also not a submerged building.
Submerged structures are generally multilevel buildings such as laboratories, shopping centers, and carparks, as well as hall-like structures such as subway stations. Underground buildings which fall clearly into other special building types such as churches or stadiums are classified with those types.