A large structure in the middle of the desert designed to hold the worst criminals imaginable.
Note that there is no barrier, but the sides are sheer walls of sand that are impossible to climb, and the entire area seems to utterly sap one's strength.
There is no hope of escape or rescue, as noone knows its location except the Kazekage.

(From the moment you get thrown in here, your stats are automatically reduced to 1. Have fun dying of dehydration. Or you know, cannibalism, whichever happens first.)