San Clemente Church in Rome must be the real-life model of the church from Ngaio Marsh’s mystery novel, “When In Rome.” Although it’s one of her later books, and not the best of them all, it’s got its points, and the mystery takes place in the Mithraic temple 2 levels below the church.
Underground Fun: European Edition – Boing Boing

What makes San Clemente special is what lies beneath. Take the stairs down from the 12th century church, and you’ll find yourself in a previous incarnation of the Basilica that dates to the 4th century. The light is bad down there, but below you can see a crappy, but passable, picture I took from that level of the church.

But you know what’s even cooler than an old church with an older church underneath it? An even older building underneath that. You can actually go further down, and further back in time, to the ruins of 1st century AD Roman buildings, which were likely the location of a temple to Mithras, a sun god whose mystery cult some scholars think may have heavily influenced early Christian ritual and belief. It’s pretty badass. Unfortunately, the lighting really sucks down there. I’ve got no photos from that level and I wasn’t able to come up with creative commons shots from other sources, either. Although the church’s official Web site has some neat renderings and a few pics that you can see. I didn’t get a guided tour of the Basilica, so I know less about its history. But it’s definitely worth a peek if you’re in Rome and love old, underground things.