
Subterranean
Geographical Context
The word 'subterranean' is often used when talking about natural or man-made objects below the Earth's surface. 
Physical Realm Focus
'Subterranean' specifically denotes things not visible on the surface, especially related to geography and structures. 
Not For People
While 'subterranean' often describes places, it's not typically used to describe people or animals. 