Petra, Jordan
While Petra may not be lost, it was certainly abandoned. The desert city flourished until an earthquake destroyed the vital water system that sustained its population, which preferred to simply move to nearby settlements that had access to essential resources. From then on the site was left to the desert, attracting only the most curious of travelers and grave robbers. Currently it is a world heritage site and one of the greatest middle eastern archaeological sites. The city is partly built from ground up, and partly carved into the red rock that populates the region. The architecture features are a fusion of Roman, Greek and native Nabataean.
