| Category | Lexicon |
| Type | geographical lore |
The Underdark is the area beneath a world's surface. Cartographers often break them into levels, the nearest to surface is Tier 1, the lowest any other number. The Underdark is sometimes called the "Lightless Realm", yet this is not entirely true. There are vast stretches of darkness, but also large areas lit by dull glows from organisms and subterranean vegetation. The forests of the Underdark are generally referred to as fungals.