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Researchers examining data from the New Horizons space probe, which passed Pluto in 2015, were puzzled by a 27-mile-long crater on Pluto's icy surface. Although Pluto is mostly covered in frozen methane and nitrogen, the surface near the crater seemed to show the presence of water ice and ammonia—both of which are associated with eruptions from ice volcanoes. Additionally, the shape and appearance of the crater did not suggest that it was formed by impact. Based on this evidence, scientist Dale Cruikshank and his team hypothesized that the crater was likely once a massive ice volcano.