The Underground is the stony belly of the world. There is no day or night cycle here, and hostile mobs are found in great numbers.
Stairs to the Underground will generally appear about 20 tiles away from the spawn point. Upon first entering the Underground, it will be completely dark except for a small circle of light that surrounds the player.
Abundant features of the Underground depend on how deep the player is. Spikes, found on all levels of the Underground, can be mined by the player to receive valuable ore. Lava is prominent on the third level of the Underground.