| Category | Lexicon |
| Type | cartography |
Regions beneath the surface are collectively called the Subterranean Tiers. They are divided into tiers (or levels) of varying thickness. They are commonly broken into a areas of depth of around 4000', such that the first subterranean tier under the surface is 0' - 4000' depth, the second subterranean tier from 4001' - 8000' depth, and so on.