A massive oil spill discovered earlier this month at Prudhoe Bay is the largest ever on Alaska's North Slope, officials said. Between 201,000 and 267,000 gallons of crude leaked from a ruptured transit line onto the snow-covered tundra, according to an official estimate of the spill Friday. By comparison, the Exxon Valdez spilled 11 million gallons when it ran aground in Prince William Sound in 1989. "I can confirm it's the largest spill of crude oil on the North Slope that we have record of," said Linda Giguere with the state Department of Environmental Conservation. She said the state began comprehensive record keeping on spills 10 years ago, following years of cursory record keeping since the trans-Alaska oil pipeline was built in the 1970s. The estimate was based on a survey conducted this week at the site operated by BP Exploration (Alaska) Inc. where the leak was discovered March 2. Workers took measurements by probing the snow covering much of the crude that leaked from the 34-inch line.