Istanbul: Sultanahmet - Yerebatan Saray (Sunken Palace), Cistern Basilica

There are 336 columns and about 12 inches of water on the floor. Wooden planks have been laid down so you can walk amongst the columns some of which are quite decorative.
One of the most famous sights within the cistern is the pedestal with the two Medusa heads carved into it: One head is on its side, the other inverted