Underworld

The Underworld is the dimension of the dead.

History
When the particle accelerator went off, one of the people it had killed went to the Underworld. He later returned as Nekron, and attempted to take The Flash's soul, in order to take his speed. The Flash used Nekron's scythe to send him back to the Underworld, and then created a magnetic field to trap him down there for approximately 2 years.