The walls of ‘Inca City’ rise near the south polar ice cap of Mars in this image from Mars Express, a European spacecraft in orbit around the Red Planet. The ‘walls,’ which are miles long and hundreds of feet high, could be volcanic rock that filled cracks in the floor of a wide impact crater. The lanes of hardened magma later were excavated by winds. [ESA/DLR/FU Berlin]
Find out more in our radio program entitled Moon and Mars.