The Underworld is the 6th layer of The Overworld mine in Mining Simulator 2. This layer of the mine begins when you reach a depth of 1,477. The layer continues until a depth of 1,836.
You can access another free chest in The Underworld lobby. This chest rewards some coins based on your multiplier & respawns every 5 hours. Reset it instantly for 375 Robux.
There is also an egg that you can hatch in this layer called the Underworld Egg.