Isidis Planitia is a plain located within a giant impact basin on Mars, located partly in the Syrtis Major quadrangle and partly in the Amenthes quadrangle. At approximately 1,900 km (1,200 mi) in diameter, it is the third-largest confirmed impact structure on the planet, after the Hellas and Utopia basins. Isidis was likely the last major basin to be formed on Mars, having formed around 3.9 billion years ago during the Noachian period, by an impactor around 200 kilometres (120 mi) in diameter. Due to dust coverage, it typically appears bright in telescopic views, and was mapped as a classical albedo feature, Isidis Regio, visible by telescope in the pre-spacecraft era.