This crater, known as Orcus Patera, is a strange, elliptical crater located near Mars's equator. Roughly 240 miles long by 90 miles wide, its elongated shape doesn't fit with standard models of crater formation, leaving astronomers with a baffling mystery.
Orcus Patera is located between two of Mars's biggest volcanoes, Elysium Mons and Olympus Mons (which is also the highest peak in the solar system). Beyond its immense length and width - it's not quite as long as the Grand Canyon, but five times as wide - it rises up over a mile above the surrounding area, and it reaches depths more than a third of a mile beneath the Martian surface.
The term "patera" is used for irregular craters formed by volcanoes, with other famous examples including Mars's Hadriaca Patera and Tyrrhena Patera. But while Orcus Patera is definitely irregular, it probably wasn't formed by volcanic activity, and that's the mystery astronomers are still trying to solve.
One possibility is that it started life as a regular, nearly circular crater created by meteorite impact. Over time, compressional forces brought on by shifting tectonic plates squeezed Orcus Patera into its modern shape. It's also possible that Orcus Patera was once two craters right next to each other, and erosion wore away the distinct region separating the two until only one weird crater remained.
However, the most likely theory is that an object struck the crater at a remarkably shallow angle, probably less than 5 degrees from horizontal. This is known as oblique impact, and it would be able to dig out such an unusual-looking crater shape. Still, there's no conclusive evidence in favor of any of these theories, and so Orcus Patera remains a mystery to astronomers until the day comes we can get out there and take a look at it up close.