Scientists say air and ground surveys have allowed them to confirm the existence of an ancient impact crater buried beneath a town in Iowa.

Researchers writing in Earth Magazine, published by the American Geosciences Institute, report an airborne geophysical survey and hydrology surveys identified a crater at 3.4 miles wide, almost five times the size of the Barringer Meteor Crater in Arizona.

"We now have three distinct data sets that all confirm the presence and geometry of an impact structure," said Andy Kass, a geophysicist with the U.S. Geological Survey in Denver.

The crater's existence was first hypothesized in 2008 when geologists examining cuttings from water wells drilled near the town of Decorah were surprised to find evidence of a previously unknown shale deposit.

"We were really lucky in this case that the shale layer was preserved within the crater, as it was eroded away nearly everywhere else," Kass said. "If you travel to Decorah, you see a beautiful town, but certainly no impact structure."

Scientists say the crater's size suggests a meteorite roughly 250 yards in diameter hit the region about 500 million years ago.

Statistically, similar-sized impactors could hit Earth's surface every 30,000 to 60,000 years, they said.