The atmospheric and cavernous underground Basilica Cistern, built in the 6th century to provide a fresh water supply for buildings including the Emperor's palace, is a well-known tourist attraction in Istanbul. It seems this structure merely scratches the surface of ancient engineering.

Wonders of Constantinople's underground water world to go on display in Istanbul
The Basilica Cistern of Constantinople
British archaeologists, working with Turkish scientists, have revealed a vast water supply system that served the Byzantine-era city of Constantinople, now Istanbul.