Meteorite hopscotched across Argentina.

A chain of craters running across part of Argentina formed during an extremely rare type of meteorite impact a few thousand years ago, according to two researchers. Instead of crashing solidly into the ground, the meteorite hit at a shallow angle, apparently breaking into pieces that ricocheted and gauged their way across the landscape.

"We've seen these [types of craters] on other planets and produced them in the laboratory, but we've never found them on Earth before," says Peter H. Schultz, a planetary scientist at Brown University in Providence, R.I., who studied the craters along with Ruben E. Lianza of LTV Aircraft Products Group in Dallas. Lianza discovered the features while flying over the Pampas region of Argentina. The two described their work in the Jan. 16 NATURE.

The researchers located 10 oblong craters strung in a 50-kilometer-long line near Rio Cuarto. The largest of the shallow depressions measures roughly 4 km long by 1 km wide. Inside the holes, Schultz and Lianza found pieces of meteorite rock, the largest about fist size. They also discovered glassy fragments, formed when heat from the impact melted the surrounding rock.

Schultz estimates the meteorite was 150 meters in diameter, and moving at about 25 kilometers per second (more than 55,000 miles per hour). To account for the line of stretched-out craters, Schultz says the meteorite must have hit at an extremely shallow angle, between 5 degrees and 15 degrees above horizontal. It either broke up before hitting the ground or after the first strike, sending a narrow spray of pieces flying ahead.

Because the craters do not yet show significant signs of erosion, Schultz believes the impact occurred as little as 2,000 years ago, a time when humans inhabited this region and may have witnessed the event.