Tartarus
Τάρταρος
The prison that lies even deeper than Hades.
Hesiod wrote that Tartarus lies as far below the earth as the sky stands above it.
It is where the defeated Titans are locked away, and where Sisyphus and Tantalus serve their punishments.
It differs from the realm of Hades in this: it is not where the dead go, but where the punished go.
SourcesHesiod, Theogony