True underground versions of biomes, such as the Ice biome or Underground Corruption, do not begin in this layer, but in the Cavern layer (except for the Underground Desert, which begins in Surface terrain and extends through the Underground and a portion of the Cavern).

The Ice biome, also known as the Underground Tundra or Underground Snow biome, is a biome that is the underground version of the Snow biome, starting at the Cavern layer and stopping at the lava layer. On the Nintendo 3DS version, the Ice biome starts at the Underground layer.