A new study shows that mysterious, steep-sided pits—one up to 600 feet wide and 600 feet deep—discovered by the Rosetta spacecraft on the surface of comet 67P/Churyumov-Gerasimenko are probably sinkholes caused by weaknesses in the ground beneath the comet’s surface. And this could offer clues into how old the comet is and how it came to be.

Scientists spotted the pits right away, when Rosetta became the first probe in history to orbit a comet last summer, but they were too busy mapping the comet’s craggy surface to think much about them. “We had no idea what they were,” says planetary scientist Jean-Baptiste Vincent, of the Max Planck Institute for Solar System Research in Göttingen, Germany. Vincent is lead