My favorite underground site in Europe hands down is the Basilica Cistern in Istanbul, Turkey. It is one of the most hauntingly beautiful places that I have ever been to. The Basilica Cistern is located just half a block away from the Hagia Sophia and the Blue Mosque and is completely underground. The Basilica Cistern was originally a cistern used to hold rainwater under the reign of the Byzantine Emperor Justinian. It is one of the largest cisterns underneath the city of Istanbul and I spent over an hour wandering around the different aisles and allowing myself to get lost in the sheer size of this underground cistern. There is still water in the cistern and the thousands of ancient columns reflect eerily in the shallow water.
Towards the back there are two Medusa heads supporting columns but interestingly enough the Medusa heads are upside down, it is said that they were placed upside down in order to cancel out Medusa’s power during the Christian Age. I would absolutely recommend a visit to the Basilica Cistern, it really is a remarkable place to be surrounded by thousands of ancient columns that have been holding up a cathedral like building underground since the sixth century CE.