To access Underworld, the Player must construct an Underworld Portal. To create the portal, one must first construct an obsidian frame, ranging in size from 4x5 meters at minimum to 23x23 meters at maximum. Once the frame is constructed, it can then be activated by placing fire within the frame. When the portal has been activated, it will emit an indigo mist.
An activated portal will allow all entities (exceptions include entities riding or being ridden by another entity) to be transported to a corresponding portal in Underworld. A new portal will be created in Underworld if necessary. The portal can be destroyed by breaking the obsidian frame, by an explosion, or by placing water or lava into it with a pail.
Underworld terrain is largely composed of Underworldrack that forms complex platforms and niches, with magma seas, lava falls, and rivers of lava being frequent. Lava sources hidden in the Underworldrack are also common, making large mining projects deadly. Varying fires across the Underworldrack are yet another hazard to the unwary player. Underworld is horizontally infinite.
Bedrock comprises the top (below y=127) and bottom 4 layers in a rough pattern. Fluorescent Rock clusters hang from ceilings, and morels grow abundantly on the ground. There are patches of hovering gravel and spirit sand. Small caves form in the Underworldrack.
Underworld has no day-night cycle and no weather. The only sources of light are fire, lava, portals, and fluorescent rock. A dim ambient light can be seen throughout Underworld, and is what light level 0 looks like. Light otherwise functions exactly the same as it does in the Overworld.
If the player dies in Underworld, they will respawn in the Overworld; any surviving inventory items will remain in Underworld.
Locations in Underworld correlate to Overworld coordinates, but horizontal Overworld distances are scaled down by a ratio of 8:1 for travel in Underworld. Therefore, traveling one meter in Underworld means traveling eight meters in the Overworld.
All items and object in Underworld function the same as they do in the Overworld, with a few notable exceptions:
- There is no way to place liquid water in Underworld without cheats.
- Using water buckets will produce steam and an empty bucket.
- Ice that is melted or smashed in Underworld will also not produce water.
- Lava flows twice as far (8 meters) and much faster in Underworld than in the Overworld.
- Wabe and leaves take a dead brown color, sinceUnderworld is considered a warm biome.
- Compasses are unable to find the original spawn point, as it cannot be set to a single point within Underworld.
- Clocks are unable to determine the position of the Sun and Moon.
- The music disc of, "Sunshine Lollipops and Rainbows" can be found here.
- The song "Sunshine Lollipops and Rainbows" will play in Underworld no matter what.
Underworld houses its own share of creatures, all of which are completely fire-proof and immune to lava. Overworld creatures like bovines and pipipis do not naturally spawn in Underworld, but they can wander or be pushed through a portal.