The map of the subterranean level of Taming of the wild
The subterranean level, or underground, is a gameplay concept in the Heroes of Might and Magic series. It was first introduced in Heroes of Might and Magic III, and allows a map to be split into two parts: one surface level, and one underground.
Usually, heroes travel between the two levels through the use of subterranean gates, but they can also use monoliths, or whirlpools. The adventure map of the subterranean level is always the same size as the surface, but usually, a lot of the space is not utilized, containing nothing but a black emptiness. Sometimes, both levels are filled with explorable land, and in a few rare cases, the subterranean level is actually more developed than the surface. The Regicide is a good example of the latter.
Some factions are more likely to be found underground, like all versions of the Dungeon, or the Fortress in the Ashan universe, as well as the Inferno faction starting in Might & Magic: Heroes VI.