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.
Surrounding the pit are patches frozen carbon dioxide. The circles in the ice is thought to where the dry ice has sublimated into gas in the summer sunshine, leaving what astronomers call "Swiss Cheese terrain".
The image was taken using the MRO's High Resolution Imaging Science Experiment, or HiRISE camera, which allows researchers to see objects on Mars that are larger than one metre (about 3 feet) in size from about 200 to 400 kilometres (about 125 to 250 miles) above.
That means the pit isn't tiny – at 50 centimetres (19.7 inches) per pixel, we're looking at a feature hundreds of metres across. Take a look on NASA's website for a hi-res version of the image.