Thunderclap is a doomed realm at the edge of End-World just east of the Borderlands and is a dry, rainless desert. It's described as a land of deserted villages, dead fields, dead soldiers, and blasted trees where the graves vomit out their dead and the clocks run backward. Roland Deschain knew he had to pass through Thunderclap to meet his mortal enemy once more.
Beyond Thunderclap are the Discordia Badlands and Castle Discordia and is the home of the Devar-toi, home of the Breakers. The only light in Thunderclap shines down on the Devar-toi and comes from an atomic sun, which runs on what is basically a giant egg timer.
Doors are used by the Wolves to travel from Thunderclap to the Callas, but they have to use a train on their return trip to Thunderclap.
Ted Brautigan, Dinky Earnshaw, and several other Breakers decided to risk crossing Thunderclap in order to reach the Callas and find their new lives.