VICTORIA ISLAND - A meteorite the size of four Wal-Mart Supercenters likely plunged into what we now know as the Delta, millions of years ago, according to a geologist and his teenage son.

The duo recently found what they believe to be a 3.4-mile-wide crater buried far beneath the asparagus fields of Victoria Island, about 15 miles west of Stockton.

The discovery was entirely by accident.

Geologist Bennett Spevack of San Diego works for a firm that was drilling in search of oil; while poring over data, he found a circular depression.

His son, Samuel, a senior in high school, took on the mystery for a science fair project. The result: A co-authored study released earlier this month by the Lunar and Planetary Institute.

What would it have been like to sit in your downtown Stockton office when this 1,000-foot-wide rock hurtled into Earth?

"At best, it would have been a bad day in Stockton," Bennett Spevack said Thursday.

That is, if bad includes everyone's clothes immediately catching fire, an earthquake damaging or destroying most buildings in town and nearly every tree uprooted by a blast of air six times stronger than the winds of Hurricane Katrina.

"Oh wow, oh wow, I can't imagine what would happen," said Graydon Nichols, whose family owns the 7,000-acre Victoria Island. Farmers were excited about the crater discovery, he said, but it's the peat soil that makes the asparagus grow so well - not some magic mineral from outer space.

The explosion topped 1,500 megatons, the equivalent of well more than 100,000 atomic bomb blasts, Spevack said. Fortunately, it happened at least 35 million years ago, during the period of time known as the Eocene Epoch, when scientists say much of the Central Valley was submerged beneath a shallow inland sea.

Herds of piglike beasts roamed the land, munching on tropical plants; rhino look-alikes butted heads while carnivores the size of coyotes swung through palm trees.

The dinosaurs already were history, likely wiped out by a meteorite that makes the Delta's own deep impact look like a pebble dunked into Lake Tahoe.

Nevertheless, the meteorite that splashed into our Valley sea must have triggered tsunamis and great temblors, Spevack said.

His rock was larger than the Barringer Meteorite, which crashed into Arizona 50,000 years ago, carving out a crater that is now a famed tourist attraction.

Barringer experts say there are 150 proven space impact sites around the world and likely many more we do not know about. Smaller meteorites weighing perhaps a few ounces rain down on the planet by the thousands each year; the big ones show up perhaps once a millennium.

Just two likely craters have been documented in the Valley: the Victoria Island crater and a smaller impact southwest of Sacramento.

The geologists plan to study cuttings from the oil wells in search of evidence of the meteorite, said Spevack, who works for Bakersfield-based ABA Energy Corp. The rock, most likely iron, was clearly blasted to pieces upon impact and eventually disappeared beneath a mile-thick layer of sediment that accumulated over the eons.

"It wasn't of the size that it would have had a worldwide effect" on life, Spevack said. "It's not large enough to cause the dinosaurs to disappear. It would have caused the town of Stockton to disappear, though."