Changes in climate can make a friendly place less welcoming. Catastrophes like volcanoes or giant earthquakes can kill a city quickly. Political or economic shifts can strand what was once a thriving metropolis in a slow death of irrelevance. After the Mississippi River flood of 1993, the residents of Valmeyer, Ill., voted to move their entire town two miles east to higher ground.

What will happen to New Orleans now, in the wake of floods and death and violence, is hard to know. But watching the city fill up like a bathtub, with half a million people forced to leave, it has been hard not to think of other places that have fallen to time and the inconstant earth.

Some of them have grown larger in death than they ever were in life.

Take the library in Alexandria. If anyplace might have had justifiable pretensions of permanence it would have been the library, founded sometime around 300 B.C. It grew under the early Ptolemaic rulers of Egypt into an enduring symbol of culture and knowledge before disappearing into the sand and sea less than 1,000 years later.

"This was the library," said Roger Bagnall, a historian at Columbia. "It influenced everybody who ever thought about building a library."

Nobody, Dr. Bagnall complains, knows how large it was - estimates range from 40,000 to 400,000 scrolls - or what was actually in it. The library's demise is equally shrouded in myth. One legend says the books burned during Caesar's conquest of Alexandria in 47 B.C., but the library was still around in the fourth century, according to historical accounts.

Dr. Bagnall thinks that simple neglect killed the library. "Books rot," even acid-free papyruses, he said, noting that there are no records of any investment in maintaining the library after the early Ptolemies.

By the time Christian mobs sacked the library and museum at the end of the century as a pagan institution, there was probably little left to destroy. "The palace quarter was pretty well wrecked by that point. Whatever had survived the rotting didn't make it past that," Dr. Bagnall said.

Later, in 642, the Arabs moved Egypt's capital to the Cairo region and Alexandria shrank into obscurity.

On the other side of the globe, in the Caroline Islands of Micronesia, a stony silence relieved only by the lapping of waves envelops the empty city of Nan Madol. It consists of almost 100 islands, built by humans and constructed of columns of basalt 15 feet long and weighing 5 tons, stacked log cabin style to make walls 25 feet high.

Local legend says that you will die if you spend the night there. But once this was home to the nobles and priests of the Saudeleur dynasty, which ruled Pohnpei's 30,000 inhabitants up until about 500 years ago, according to William S. Ayres, an archaeologist at the University of Oregon.

Dr. Ayres said that Nan Madol was constructed out on the reef, starting about 1,500 years ago, partly because people had been living out there for hundreds of years to have easy access to the sea, and, perhaps more important, to better commune with ocean deities.

The columns for the walls were quarried on Pohnpei, he said, and floated out to the reef on rafts, about 500,000 tons in all over the 1,000 years of construction, Dr. Ayres has estimated. While most of the islands were living quarters for priests, others were given over to special purposes like making canoes, preparing coconut oil or, most grandly, burying royalty in tombs with courtyards surrounded by 25-foot walls.