This is the Pont du Gard, an ancient Roman aqueduct built over the Gardon River in Provence. Ken and I visited the monument, which is just outside the city of Nîmes, back in 1989. It was my first time there. Back then, tourists could walk across the top level at their leisure and we did. I don't think I'd do it today. As you can see from the photos, the top level is kind of narrow and has no safety rails whatsoever. Just beneath the top layer of stone is where the water flowed and visitors could also walk through the tunnel that carried it.
Sometime after 1989, the top two levels were closed to visitors. You can imagine why. But I read that people can sign up for a guided tour that takes them to the top. The aqueduct is an amazing structure, especially in that it's still standing after more than two thousand years. Almost no mortar was used in its construction, another amazing fact.
I originally posted these photos on the blog back in 2007 (click on "southwest france trip" in the sidebar and scroll through). I've reworked them and made them bigger for today. Enjoy!