The Underworld is a realm directly beneath bedrock level. It is home to races such as the the Dark Dwarves and the Fire Dwarves
This world is predominantly made up of bedrock and is a cave system similar to The Nether. Ores such as Undergold may be found, but these are hard to mine as they are encased in bedrock.
There is flora and fauna such as Mooshrooms, and mushroom forests. There are also large lakes of water and lava.