The Great Sand Sea
used to be where The Ancient Empire of Netheril was located... Think of magicians powerful enough to uproot an entire mountian, flip it over, levitate it, and then construct a city atop it... The largest (I think) was Karsus, and it was a great many miles across. Only one of those cities is known to have survived the Fall of Netheril, but a great many ruins would be ripe for the picking...