Lost cities are places, once prosperous and well-populated, which have since disappeared for some reason, or dwindled into near nothingness. There was an Arabian city named Ubar, which became abandoned with changes in trade routes, and its location was forgotten for some centuries: it was rediscovered in 1992 by satellite photography revealing the traces of the ancient tracks.
Cities which historians aren't sure what happened to include the Colony of Roanoke. In August 1590, John White returned to the former English colony, which had housed 85 men, 17 women (two of them pregnant) and 11 children when he left, to find it completely empty.
Malden Island, in the central Pacific, was deserted when first visited by Europeans in 1825, but ruined temples and the remains of other structures found on the island indicate that a small population of Polynesians had lived there for perhaps several generations some centuries earlier. Prolonged drought seems the most likely explanation for their demise. The ruins of another city, called Nan Matol, have been found on another polynesian island, Ponape
Cities have been destroyed by natural disasters and rebuilt, again and again, but the destruction has occasionally been so complete that they were not rebuilt: the classic examples are the Roman cities of Pompeii and Herculaneum, buried with many of their inhabitants in a catastrophic flow of volcanic ash from an eruption of Vesuvius. A less well known example would be Akrotiri, on the island of Thera, where in 1967, under a blanket of ash, the remains of a Minoan city were discovered. The volcanic explosion on Thera was immense, and would have had disastrous effects of the Minoan civilisation. It has been suggested that Plato may have heard legends about this, and used them as the germ of his story of Atlantis
Less dramatic examples of the destruction of cities by natural forces are those where the coastline has eroded away. Cities which have sunk into the sea include: the once centre of the English wool-trade, at Dunwich, England, and the city of Rungholt in Germany.
Cities are also often destroyed by wars. This is the case, for instance, with Troy and Carthage, though both of these were subsequently rebuilt. The various abandonded capitals of the middle east are an interesting case: Persepolis was accidentally burnt by Alexander the Great, while Babylon was abandoned in favor of Ctesiphon, which was in turn abandoned in favor of Baghdad, though all these are fairly close together.
Some of the cities which are considered lost are (or may be) places of legend such as the Arthurian Camelot, Lyonesse or Atlantis, whilst some, such as Troy, having once been considered to be legendary, have been subsequently discovered to have a basis in reality.