You'd think NASA's Mars Reconnaissance Orbiter (MRO) has seen everything there is to see on the Martian surface in the 11 years it's orbited our nearest neighbour, but a snapshot taken over the planet's South Pole has revealed something we can't explain. While the planet's entire surface is pocked with various depressions and craters, a vast pit spotted among the "Swiss cheese terrain" of melting frozen carbon dioxide appears to be a bit deeper than your average hole, leaving astronomers to try and figure out what made it. A lot of things can make holes in Mars' rocky terrain: more than half a million meteorite impacts have left craters; collapsing lava tubes have created deep pits; ancient floodshave gouged out giant chasms; and volcanic activity has melted ice to leave funnels. Occasionally the MRO will come across an odd feature that poses a fun mystery to solve, such as this shallow, circular depression seen earlier this year. But there's nothing so shallow about this newly