|Residents||Skeletons, Lord Garmadon (Formerly), dead things|
The Underworld is the dark deep home of the Skeleton army.
Always inhabited by the Skeleton race, Lord Garmadon was banished there by Sensei Wu. After building himself a Dark Fortress, Lord Garmadon put himself as the true power in control of Samukai, Skeleton King. They found that traveling at very high speed can let you phase from Ninjago to the Underworld. Knowing this, they travelled in Skull Motorbikes, Turbo Shredders, ATVs, and Monster Trucks to create disaster above. All this time, Garmadon was manipulating the Skeletons.
After failing to get the Scythe of Quakes, Samukai confessed to Garmadon, and was shocked to learn that everything was going well. Wu took the Sword of Fire into the Underworld to take it from the Fire Temple. As the ninja flew down on dragons, Wu was defeating Skeletons. The ninja were spotted by Nuckal, but used the Tornado of Creation to escape. Samukai beat Wu, and took the last Golden Weapon. The power ripped Samukai apart, and opened a portal for Garmadon to finally escape. The ninja left.
There are several floating islands where the Skeletons live. There are nicer places for those that have died. All the plants are petrified. There's a home called Grave Island.
Every dead animal and person live in the Underworld, along with the Skeleton race.