Dr. Blom explained that the space imagery has three important advantages for such exploration. First, the images are obtained in digital form and so can be manipulated by computers to bring out subtle detail. Second, a single image can cover a vast area, revealing regional patterns that might not be obvious in close-up pictures.

Octogon With Towers

  Third, the images in nonvisible wavelengths exposed in detail disturbances in the surface geology. Soil along the caravan routes, for example, has been beaten down to finer-grain particles than on the surrounding rocky surface. The difference often does not show up in regular photography. Even on the ground, Dr. Blom said, only an experienced camel driver might be able to make out the track.
  From this combination of space imagery, the analysts mapped a network of caravan trails converging on Ash Shisar. The first ground reconnaissance, conducted in the summer of 1990, uncovered artifacts along the tracks indicating that this had been part of the frankincense trade route. Several other possible sites for Ubar in the vicinity were ruled out.

  Last November, the full team of explorers, archeologists and geologists returned and, using satellite navigation equipment, found their way to the tracks leading to the well at Ash Shisar. Thomas had been wrong to think nothing more than a "rude fort" had once stood there.
  After weeks of digging, Dr. Zarins said: "This site, the structure of the thing, is quite remarkable. It is octagonal in shape with eight identifiable towers, each of which can be estimated to have once been some 30 feet high, with adjoining walls and interior rooms. Nicely plastered facing as been found on one of the towers."
  Dr. Zarins said the remains appear to predate every known site in southern Arabia associated with the frankincense trade. Roman, Greek and Syrian pottery has been excavated, with some of the Syrian material dating back more than 4,000 years. The expedition is still waiting the results of tests to date the pottery more precisely.
  The archeologist said it was difficult to determine now when the city sank into the sands. But he said the structures were built around a water reservoir in a limestone cavern and collapsed from their own weight.
  Enough has been revealed, expedition leaders said, to imagine the splendor of this major city on the frankincense route, probably the "imitation of paradise" that was Ubar.
  "It must have really been a splendid sight out in the desert six or eight days from the last water," Mr. Hedges said. "You can see how it took on a mythic quality."