TOKYO — A crater from a meteorite impact more than 20,000 years ago has been discovered in the Japanese Alps, an amateur geologist announced this week. The crater is the first found in this country.

Masao Sakamoto said the crater stretches 900 yards in diameter and spreads out across rugged, heavily forested land in Nagano prefecture (state), about 100 miles west of Tokyo.

Sakamoto, who announced his discovery at an academic symposium earlier this week, said it went largely unnoticed because only about 40 percent of the crater is visible.

"If it had been a clear, pretty circle, it would have been obvious that was a crater," Sakamoto told The Associated Press on Friday. "Everyone around here is really surprised by this."

Sakamoto said analysis of the soil at the site indicates a meteorite about 45 meters (150 feet) across smashed into the area about 20,000 to 30,000 years ago.

Sakamoto, an elementary school teacher, said he studied the crater — located in the town next to his — for 20 years before he was able to determine it had been formed by a rock from outer space.

At first, Sakamoto thought the mountain ridge and basin might have been formed by a volcano, a fault, or even sculpted out by a glacier. But the soil he found didn't match any of those theories.

After studying craters in the United States and Europe, he discovered some of them had similar features to his ridge — including a mysterious uneven stretch of valleys and hills in middle of the woods.

Quartz found on the site was then proved to have been formed as a result of the intense heat created by the impact of a meteorite, Sakamoto said.

Sakamoto presented his findings at a symposium sponsored by the National Institute of Polar Research, which is involved in geology and geophysics studies. The announcement was front-page news in Japan.

Sakamoto said he hopes the finding of an impact crater in Japan will allow his colleagues easier access to carry out field studies in meteorite research.

"The biggest honor is to have spurred such opportunities in Japan," he said.