There is a huge hollow next to the mosque, it is the Cistern of Aspar. Known in Turkish as Sultan Selim Çukurbostanı ("sunken garden of Sultan Selim"), it was a Byzantine open-air water reservoir in the city of Constantinople. When I first saw it there were trucks parked there, and on some days it seems to hold a cattle market. It later was turned, like other cisterns, into a fine sports complex.