A secret base built in a crater on the planet Mars? Google Mars shows a 4km width crater on Mars in which a large closed building structure seems to have been built.
Visible are two large rectangular black areas, separated by a rectangular pillar and in front of the structure on the left, another building.
But before we assume it is a secret base, there is also the possibility that the structure is nothing more than a compression artifact caused by digital compression software, giving it a pixelated look.
Though, surrounding craters do not show similar structures which makes it possible that the huge structure in this crater is not a compression artifact but a hangar with two (black) entrances to hold spacecraft.
Coordinates Google Mars: 10°47'20.46"N 130° 0'41.91"E