The Basilica Cistern in Istanbul is open daily from 9am to 6:30pm, with a short closure from 6:30–7:30pm while staff switch over between sessions. It’s one of several hundred underground cisterns beneath the city, built in the 6th century under Byzantine Emperor Justinian I to store water for the Great Palace.
Inside, roughly 336 marble columns support the vaulted brick ceiling, lit dramatically for atmosphere. The most photographed features are two carved Medusa head column bases in a far corner, thought to be reused from an earlier Roman structure.
Most visitors spend 30 minutes to an hour walking the platforms; arriving right at opening or in the late afternoon avoids the worst of the cruise-ship crowds, and booking a ticket online in advance skips the entrance line.