The Theodosius Cistern is a 5th-century Byzantine water reservoir built to store water brought into Constantinople by the Valens Aqueduct. It was rediscovered in 2010 after a later building above it was demolished, and has since been restored as an atmospheric underground sight.
It is smaller than the Basilica Cistern, but also calmer, better lit, and easier to take in at a slower pace. Its central Sultanahmet location makes it a good companion stop to the Hippodrome, Blue Mosque, and other Byzantine-era remains nearby.