Dreamed of a flooded trench with a mosaic below

I'm standing in a trench with water up to my waist. It's perfectly clear, and I can see a mosaic floor beneath my boots — all blues and greens, a sea scene, I think. Every time I try to reach down to touch it, the water rises a little higher. I know it's somewhere in southern Italy, but I can't name the site. The strange part is that the water isn't cold. It feels like standing in the sun.