Giant pits nearly 600 feet deep on the surface of Rosetta’s comet are likely caused by massive sinkholes, scientists say. In a paper published in the journal Nature, researchers describe 18 mysterious depressions on the speeding comet that they say could only occur by sinkhole collapse. On average, each of these quasi-circular holes are as wide as two football fields placed together, and some are as deep as the Washington monument is tall. “That tells you something about the scale of these things,” said Dennis Bodewits of the University of Maryland and a co-investigator on Rosetta’s OSIRIS instrument.