SAN FRANCISCO (AP) _ Scientists think they found the smoking gun to show that asteroids wiped out the dinosaurs: a 110-mile-wide crater in Mexico possibly gouged by a mountain-size rock smashing to Earth.

The crater near the Yucatan Peninsula town of Chicxulub may have been ''ground zero'' for the collision that some scientists believe killed dinosaurs 65 million years ago, University of Arizona scientist Bill Boynton said Tuesday.

The collision, with the estimated power of 70 million one-megaton bombs, could have kicked up 3-mile-high tidal waves and a sparked a worldwide firestorm.

''We think we have found the smoking gun, but there may be other guns and maybe a firing squad,'' Boynton said during a meeting of the American Geophysical Union.

Boynton and other researchers said another impact crater 22 miles wide near Manson, Iowa, and possible craters under the Caribbean near Colombia and Cuba point to the possibility a shower of asteroids or comets caused the extinction.

The Yucatan crater Boynton and his colleagues theorize was caused by an asteroid striking shallow Mexican waters is now on dry land, buried by 65 million years of sediment.

The study by Alan Hildebrand, Boynton and others was the latest of more than 2,000 in the past 12 years that examined whether asteroids, huge volcanic eruptions or slow climate change sent the dinosaurs to oblivion.

In 1978, scientists found deposits of iridium metal in 65-million-year-old rocks around the world. Iridium is rare on Earth but less rare in asteroids and comets. The killer-asteroid theory was born because an impact would have spread iridium dust around the globe.

The theory is that a 6-mile-wide asteroid or comet - or a comet shower - smacked into the Earth, kicking debris into the atmosphere and causing radical environmental damage that wiped out the dinosaurs and up to 75 percent of all species.

''We wouldn't be here if it hadn't been for the extinction of the dinosaurs,'' said geologist Walter Alvarez of the University of California at Berkeley. ''They were ruling the place. Mammals were just running around underfoot until the impact killed the dinosaurs and let us evolve.''

Some researchers think the impact or impacts clouded the sky with smoke, dust or debris that blocked out sunlight and wiped out food supplies. Others have theorized, somewhat in jest, that changes in vegetation upset the dinosaurs' diet, leaving them fatally constipated.

Other possible deadly effects of asteroids hitting Earth include acid rain, giant tidal waves, dark freezing weather, global warming by the greenhouse effect and a worldwide firestorm that broiled creatures in their tracks.