There is now one large supercontinent, "Pangea II", the bulk of which is north of the equator. The centre of this continent is a huge extreme desert, with no rainfall to speak of. A single world ocean has a major effect on weather patterns. Water travels westward around the equator, being warmed by the Sun as it does. In today's world there are continents in the way of this equatorial current, that deflect it to the north or south before it has chance to get too warm.
The Central Desert is a wilderness of sand and gravel, without clouds or rainfall. At night, this desert is as cold as the Earth has ever been.
Beneath it, there is a labyrinth of limestone caves. Constant rain on the seaward slopes soaks into the rocks and accumulates in the caves.
Average temperatures range from over 50oC in the summer to -30oC in the winter. The only water is from subterranean springs.