YOKOHAMA, Japan: An estimated 3.9 million gallons of crude oil spilled in Tokyo Bay today after a Panamanian supertanker ran aground. The spill is considered the worst in Japan's history. Authorities said the 3 1/2 mile slick is slowly drifting north towards Tokyo and has already sickened at least 15 people on shore. There is considerable fear that a spark could set the spilled crude ablaze. The spill occurred at approximately 10:30 a.m. Japan time and was stopped about 1 1/2 hours later. Emergency crews worked frantically to contain the slick with oil absorbent mats and oil dissolving solvents. The accident reportedly occurred after the 147,012-ton tanker scraped a reef in shallow waters about 22 miles southeast of Tokyo and about four miles east of Yokohama. The contact apparently tore holes in two oil tanks near the starboard bow. Officials said the captain may have entered the shallow waters in an attempt to avoid an oncoming vessel. The supertanker, known as the Diamond Grace, was carrying 75.5 million gallons of crude to Kawasaki from the United Arab Emirates.