By Paul Rincon
BBC News science reporter, in Houston, Texas
Evidence for what may be a large and relatively recent impact crater has been found off the coast of Antarctica.
The tektites are just a few ten-millionths of a metre across
Scientists say the evidence, if correct, points to a space rock some 5km across having crashed into the Ross Sea about three million years ago.
This could have generated a huge tsunami, according to a member of the team investigating the collision.
Details were reported at the Lunar and Planetary Science Conference in Houston, Texas.
Researchers from the Lamont-Doherty Earth Observatory in New York have been studying a 100km-wide depression, known as Bowers Crater, under the Ross Sea.
Team members examined cores drilled from around the area to look for evidence of an impact.
In the cores, they found microscopic glassy grains shaped like teardrops, spheres and dumbbells which are collectively known as tektites.
Some scientists believe these are created when rock fragments are hurled high up into the atmosphere by the impact of a large meteoroid or asteroid, and then partially re-melt as they fall back to the ground.
Other glasses were also found. These are thought to have been formed by cooling of the melted rock and sediment. Similar glasses can be formed through volcanism, but the Ross Sea specimens seem to have a distinct structure under the microscope.
The findings alone do not prove there was an impact in the area a few million years ago, but team member Dallas Abbott says she hopes to search the core material further for a mineral called shocked quartz.
This type of quartz can be distinguished from normal quartz by characteristic lines visible under the microscope which are thought to be formed by the intense pressure of an impact.
The presence of this mineral is considered most diagnostic of a space collision.
Dr Abbott told the BBC News website that an impact in the Ross Sea would have generated a "pretty big tsunami".
The waves could have crashed against the shores of South America; but, she added, the geological history of that continent made it unlikely that evidence of this event would be found.