The Basilica Cistern
An amazing feat of Byzantine engineering, the Basilica Cistern in Istanbul is a vast (100,000 square feet) underground water cistern built in the 6th century. There's just a foot or so of water in it now, which makes for interesting reflections from the 336 marble columns.
The amazing thing to me is that for a century after their conquest of Istanbul, the Ottomans did not know of the existence of the cistern. They rediscovered it when they found people lowering water buckets into it from their basements.
The cistern has a remarkable number of very large fish - apparently put in by the Romans as a sort of "canary in the coal mine" to make sure the water quality was good.