A fiery extraterrestrial object more than a mile thick apparently smashed into the Atlantic Ocean southeast of Nova Scotia 50 million years ago, exploding with a force greater than thousands of nuclear bombs, a geologist reported yesterday.

Lubomir Jansa said the impact must have pushed a towering wall of seawater toward Nova Scotia 125 miles away and rocked Earth with the force of an earthquake that would rank as a "great" one in modern times.

"I would say an atomic bomb would be just for laughs if you would try to compare it," Jansa said in a telephone interview from the Bedford Institute of Oceanography in Dartmouth, Nova Scotia, where he works for the Geological Survey of Canada. "It was a big bang."

He said his associates calculated that the impact of the giant fireball into the sea probably equalled the explosive force of all the nuclear weapons in the superpower arsenals today.

Jansa and colleagues said the object that hit the ocean was either a stony meteorite or the nucleus of a comet with an estimated diameter of 1.2 to 1.8 miles.

What remains today in water 370 feet deep is a crater at least 28 miles in diameter and 1.7 miles deep, Jansa said. He said the ocean at the time was hundreds of feet deeper and the water at the impact site probably opened momentarily and was vaporized.

Although craters formed by extraterrestrial objects have been identified around the world on land, this is the first report of such a submarine crater. Scientists, however, believe many oceanic impacts occurred, because 70 percent of Earth's surface is covered by water.

They said the crater was identified by seismic surveys conducted for offshore oil exploration.

Like large craters on the moon, the submarine crater has a central peak more than a mile high formed by rock rebounding from the impact. The crater is partially filled by crushed and malformed rock known as breccia.