Oil exploration work in California's Central Valley region has uncovered a possible space impact crater.
The 5.5km-wide bowl is buried under shale sediments west of Stockton, in San Joaquin County, and is thought to be between 37 and 49 million years old. Researchers are continuing to analyse cuttings from oil exploration wells drilled in the structure.
Details of the discovery were presented at the Lunar and Planetary Science Conference in Houston.
BBC NEWS | Science/Nature | 'Crater' spied under California