Robert Hargraves was walking with friends through a valley in southwestern Montana two years ago when something caught his eye.

To anyone else, it would have been just a funny-shaped rock - if they noticed it at all.To Hargraves, a Princeton University geology professor, it was proof that a meteorite slammed into Earth near where he was standing, leaving a crater some 40 miles wide - one of the largest ever found.

Most people usually notice things like big craters, but Hargraves says this one went undetected for thousands of years because, quite simply, it isn't there anymore.

"One normally thinks of meteorite sites as a big hole in the ground, but that's a very young feature," Hargraves said in a phone interview from his office at Princeton. "What we have in that valley is extremely old. There's nothing remotely resembling a hole in the ground."

Hargraves said he believes the meteorite exploded into the Earth probably 300 million to 1.5 billion years ago and has been distorted and covered by years of erosion and seismic movement.

"Your state has an awful lot of geological activity," he said.

The funny-shaped rock Hargraves found in 1989 is a "shatter cone," a triangular shard of rock formed by the force of a meteorite hitting the Earth. He likened the cones to the small triangular glass shards formed when a bullet is fired through plate glass.

Since finding the first shatter cone, Hargraves and other geologists from Princeton, Stanford University and the U.S. Geological Survey, have returned to find dozens of the small rocks scattered all over Beaverhead County and even in Idaho.

Hargraves said it was pure luck that he spotted the first rock. "Not every geologist will immediately recognize a shatter cone," he said. "I was just traveling through the area looking at rocks. My eyes fell on this thing and I immediately knew what it was."

Hargraves said he had seen the same rock formations while studying a meteorite crater in South Africa years before.

But even with his experience, Hargraves said, he's not an expert on meteorites and asked for assistance from geologists who have done a lot more work with meteorites.

"I usually don't work with (meteorites) at all," he said. "But this has rekindled my interest."

A group of about 20 geologists is scheduled to meet in Dillon this summer to study the area and try to determine exactly where the meteorite hit. Hargraves said he's trying to get a federal grant so he can stay in Montana for at least a month to study the area.

Hargraves said one goal for the geologists will be to prove that a meteorite actually landed in the area south of Dillon.

Geologists doubt they will ever find a trace of the meteorite, since most disintegrate on impact. And since there's no large crater, Hargraves said the geologists will be very busy searching for proof.

If the area proves to be a meteorite site, Hargraves said, it will probably rank as one of the 10 largest in the world. One of the best-known meteorite strikes, Meteor Crater, Ariz., for instance, measures just over a mile wide.