It was shortly after 7 a.m. yesterday, and the Jacksons were in the home stretch of a 15-hour drive from Chicago when the car radio announced that a water main had ruptured, sending about a million gallons of water spouting into the air--at the only entrance to their Montgomery County neighborhood.

"We said, 'Oh God,' " a tired and sweaty Leslie Jackson said as she and husband George toted their own and two daughters' luggage from the car, which they were forced to park nearly a mile away as repair crews labored at Briardale and Shady Grove roads.

The Jackson residence, and about 250 others in the Derwood neighborhood, lost water service for more than 20 hours--from 7:30 p.m. Tuesday until 3:45 p.m. yesterday. Workers employed by a Bell Atlantic subcontractor to lay a fiber-optic cable had mistakenly punctured a 24-inch concrete water main, according to Marjorie L. Johnson, a spokeswoman for the Washington Suburban Sanitary Commission.

Michel Daley, a Bell Atlantic spokesman, said he could not confirm that a subcontractor caused the accident. "Until I talk to the construction manager, I won't know conclusively," he said.

The break caused a geyser--up to 20 feet high--to erupt in Briardale Road. Officials closed the only entrance to the middle-class, tree-studded subdivision, which snarled traffic throughout the area.

As work crews dug a crater to replace the damaged pipe yesterday, residents were allowed to drive out of the neighborhood but not back in, a step necessary "to allow WSSC workers to complete their repairs," police spokeswoman Joyce Barrow said.

Residents attempting to return to their homes from work or from trips to fetch water were forced to fight for parking spaces along busy Shady Grove Road or in nearby subdivisions.

"So you don't have water, you go to get some, but you can't get back in," said Mark Lynch, a Derwood resident who had to walk back to his house with five gallons of water he had purchased at a nearby Giant supermarket.

By midday, a crew from the county's Department of Liquor Control had arrived with about 120 gallons of water for residents. Luciano Prologo, one of those bringing the water, said that the county was offering two gallons per person and that within an hour, less than 25 gallons remained.

In the aftermath of the break, Briardale Road looked like a dry riverbed, covered with light brown mud and rocks carried by the geyser. Mud also flowed down a hill sloping from the intersection to a creek bed, prompting the county Department of Environmental Protection to organize a cleanup.

Some residents said they were upset that they had to learn what was happening from television news programs, instead of from WSSC or county officials. One woman, Mary Childers, said the incident was "so sudden."

"There was no warning. There was so much chaos," said Childers, who used water from her backyard pool to fill her toilet so she could flush it.

John Howes Jr., 11, stood near the hole, recalling the "phenomenal" sight of a fountain of water rocketing into the air. "It was like a brown, dirty water geyser. It was just shooting up. Rocks were just shooting all over," said John, whose trip with his dad and a cousin to Little League tryouts Tuesday evening was blocked by the water.

Chris Schneck, 35, said he phoned his boss to say he would be late. His wife, Gisela, took sick leave. Left without water, the Derwood couple said they hopped in their car--which had been left outside the subdivision the night before--and drove to a private club near Darnestown to take showers.

John Ricker, a Navy employee moving to Maine, said his moving company couldn't reach his home this morning. "It screws everything up," Ricker said, adding, "It's only really nasty because you can't flush."

Staff writer Scott Wilson contributed to this report.