Cavern + ancient ruins

We were exploring a huge cavern with a tour guide. At first, it was all natural cavern stuff, but as we got further in, we started to see the ruins of Greco-Roman style columns and related structures. As the tour guide showed us one spot, which was mostly natural rock, he showed how, if you climbed a few feet up, you could see more ruins. In fact, a square window in the back wall, which had previously been sealed with rock or cement, was somehow now open, and from the perspective a few feet up, I could see through the window and down into another huge chamber that continued downhill for quite a ways. I could see tons more columns and other ruins, all colorful with paint that I wasn't used to seeing on old, weatherworn ruins.