| Category | Lexicon |
| Type | cartography |
Subterranean regions are collectively referred to as the Subterranean Tiers. These are divided into tiers (or levels) of varying thickness. They are typically segmented into areas of approximately 4,000 feet in depth, with the first subterranean tier extending from 0 to 4,000 feet, the second from 4,001 to 8,000 feet, and subsequent tiers following this pattern.