Largest geological feature discovered from space

The Richat Structure in the Sahara Desert of Mauritania is an almost circular, multi-ringed sedimentary basin with a diameter of around 50 km (30 miles). Originally thought to be an impact crater, it is now believed to be the eroded remains of an uplifted sedimentary dome. This distinctive 'bullseye' was discovered from orbit by US astronauts Jim McDivitt and Ed White during the Gemini IV mission in June 1965. While features on Earth have been discovered using remote sensing and spacecraft data, the Richat Structure has the distinction of having been discovered by the eyes of the astronauts.