An underground cathedral.

Another from inside the Piscina Miribalis, the largest Roman cistern still in existence today. I loved it down there. A very atmospheric and eerie place. It's not that easy to get into despite advertising opening times. I found the folllowing in a guide and it is pretty accurate. Especially about actually getting in. 

"In order to visit, you must first call a number and make an appointment. Then go to the cistern at the agreed upon time, walk down a nearby block, and call out asking the locals: “Piscina Mirabilis?” They will point. Follow the fingers until you reach an apartment complex where a woman comes out with her toddler – and a key in hand. The woman walks with you to the cistern and opens the doors, but she refuses to go inside. Instead, she waits at the top while you explore on your own, trekking down steep metal steps and into a mossy cavern. 
Constructed in the Augustan period, the cistern measures 70 meters long, 25.5 meters wide, and 15 meters tall. Dug into the tuff rock, it has two entrances. The first is by way of the metal stairs. The second is on the opposite end, but the tuff stairs currently lead only to dirt. A middle nave lies one meter below the rest of the structure and once served as a decantation pool for periodic cleansing and emptying of the cistern. A thick layer of waterproof cocciopesto or signinum once covered the cistern walls. They were made of broken tiles mixed with mortar. The water capacity could reach 12,600 cubic meters and the Romans created hydraulic machines on the roof terrace that pumped the water."