As if getting around the Capital Beltway, with its daily traffic jams, dangerous curves and unsavory merges, weren't challenging enough, drivers were stalled for more than a mile on the outer loop in Virginia yesterday after a 10-foot-wide section of road fell in on itself.

The sinkhole was roughly the width of a highway lane and four feet deep. It looked as if something very large had punched a hole in the earth, and it could easily have swallowed several smaller models of cars.

It was spotted about 7 a.m. in Alexandria near the Telegraph Road interchange by workers on the Woodrow Wilson Bridge project, which includes that portion of the Beltway. Workers inspected the hole and found that it extended under a second highway lane, so both were repaved about 10:30 with a fast-drying form of concrete.

Meanwhile, cars squeezing into the two remaining Beltway lanes backed up more than a mile, to the exit for the Eisenhower Avenue Connector. They trickled through until about 2:30 p.m., when the concrete finished setting and the lanes were reopened.

Sinkholes form when the ground degrades, and spaces and caverns develop beneath the surface. When the cavity in the earth gets too big to be supported, it collapses.

John Undeland, spokesman for the bridge project, said that engineers do not know exactly why the sinkhole formed but that the soil throughout the area is not very strong.

"The whole area through there is weak soil," Undeland said. "It doesn't support the roadway very well."

In fact, workers are strengthening the soil in the area as part of the bridge project, one reason they were able to patch the hole so quickly, Undeland said. Another sinkhole formed on nearby Church Street two years ago, he said.

No cars drove into the sinkhole, and no one was hurt by it, Undeland said. Its only victims were a few more delayed Beltway drivers. So it could have been worse.

"Fortunately, everything opened up at 2:30 before everyone went to the beach," he said.

The sinkhole opened on the outer loop of the Capital Beltway near Telegraph Road.