From The Heroes of Might and Magic III wiki

Every map can have one or two layers- always a surface, and sometimes an underground as well. Heroes can move from on to the other through Subterranean Gates, Monoliths, Whirlpools, or by Town Portal. There is nothing that distinguishes one from the other, but the Terrain in the Underground is usually made up of Subterranean and Rock, while the other types are found on the surface.

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