Just one week before it was slated for demolition, a waterfront house on West Tisbury's North Shore burned to the ground yesterday in a blaze that kept firefighters from three towns working for nearly six hours straight.

The house was owned by seasonal resident Dirk Ziff, one of the heirs to a magazine publishing fortune. The house was unoccupied.

Investigators are still searching for clues to the origin of the fire, which was first spotted yesterday at about 9:10 a.m. by a construction crew working on a nearby property.

"The cause is totally undetermined. It's just wide open," said Tisbury assistant fire chief James Rogers, who is leading the investigation. "We're not going to dig out the scene until Friday or Saturday."

West Tisbury fire chief Manuel Estrella 3rd told the Gazette yesterday there was no chance to save the house.

"It was fully involved when we got there, 90 per cent of the house was gone," he said. "It could have sat there smoldering all night. It was very hot. We didn't worry about the house. We were worried about the surrounding grounds and houses."

Yesterday, witnesses reported being able to see plumes of smoke from the Steamship Authority wharf in Woods Hole. West Tisbury police Sgt. Daniel Rossi said he saw black smoke as soon as he neared the agricultural hall off State Road.

The house is located on Paul's Point, just east of Cedar Tree Neck Wildlife Sanctuary.

Firefighters said they were grateful the fire hadn't struck on Wednesday, when wind speeds across the Island topped 50 miles per hour. Still, the steady wind coming from the shore yesterday did a good job of feeding the fire.

"The wind was coming directly off the water and into the house," said West Tisbury assistant fire chief James Branch. "It was blowing the fire inland. There were quite a few little spot fires, but some civilians had gone to help put them out. It was a lifesaver. It was blowing pretty good when it started."

Chief Estrella said that the entire front of the house facing the water was glass and had quickly burned away.

"They were going to tear down the building on Thursday," Chief Estrella said. "They had some people in there doing some cleaning. There were pictures on the wall bought from Allen Whiting - they still had some valuable stuff in there."

Mr. Ziff could not be reached for comment; fire officials said he is traveling somewhere in Europe.

According to Martha's Vineyard Land Bank records, Mr. Ziff paid $10.4 million for the property in 1999. The 3,000-square-foot house had been built in 1974 and sat on 12 acres.

The new house Mr. Ziff is planning will be almost four times the size. The building permit filed in West Tisbury town hall calls for an 11,000-square-foot main house, a 2,300-square-foot guest house and an in-ground swimming pool. The total estimated construction cost is more than $6.3 million, according to the permit application.

The fact that the house was about to be torn down was common knowledge among the firefighters, but it didn't appear to dampen any of their efforts.

A major part of the firefighting operation was set up off-site at Seth's Pond, where a Tisbury fire department pumper engine had backed up right to the pond bank on the public beach.

All morning and into the afternoon, a brush-breaking truck from West Tisbury and a tanker from the Chilmark fire department drove back down the long dirt road to fill up their tanks with pond water.

Assistant West Tisbury fire chief John Early estimated that the crew drained close to 80,000 gallons of water from Seth's Pond.

It was a quite a scene on a sunny September day as some swimmers - a pair of elderly women and a mother with a young boy - decided to spend the morning at the pond beach, even with all the commotion happening just a few feet away.

Crews had dismantled part of the split-rail fence to allow the Tisbury pumper to roll onto the beach. The trucks would roll down the road and turn on their sirens as they entered Lambert's Cove road.

Firefighters would quickly link up hoses between the two trucks and give the signal. When water came shooting out the top of the tanks, they'd call back over the radio, "Twenty-one shut down."

One Tisbury firefighter handling this detail succumbed to heat exhaustion and was treated by EMTs on the scene.

The only other casualty of the day was the West Tisbury fire department's 1976 tanker truck, which broke down along the dirt road leading up to Mr. Ziff's property. The breakdown of a tanker that can hold 5,000 gallons meant other vehicles had to do extra work. Chilmark's truck holds 2,500 gallons, and the West Tisbury brush-breaker can carry only about 600 gallons, according to firefighters.