It wasn't until the 1980s that archaeologists, using NASA satellites and super-radars, located a network of camel roads leading to the remains of Ubar situated deep in the Rub' al Khali desert in the Arabian Peninsula. The records they discovered indicated the area had been a thriving hot spot. For about 5,000 years, people from Persia, Greece and Rome flocked there for one of the major commodities of yesteryear: heroin. Just kidding -- it was frankincense.
And then one day, all the hippies flocking their for their sweet frankincense fix returned saying the roads leading up to the city just sort of ... ended. The city had disappeared.
Sometime between 300 and 500 AD, the city collapsed into a sinkhole, which then collapsed into a cave.
"Well at least there's plenty of Fraggles to eat down here."
The reason why the region was such a popular hub in the first place was that they had the fantastic luck of having access to water in the middle of the desert. The Ubar water came from an oasis, which just happened to come from a limestone cavern. So, as people drew water, year after year, for 5,000 years, the cave beneath their feet was getting dryer and dryer. Without the water supporting the limestone, the cave roof got as brittle as your grandma's hip bones. Then one day, the cave collapsed and what was once a thriving oasis turned into a gaping, city-sized maw of death.