Sidenote here, the 500ft rule does *not* say 500ft above, it says 500ft clear - by my understanding, technically that *could* be 500ft laterally, and 1ft off the ground - if you're very sure nobody is within 500ft, you're technically OK.
The law actually says "person or *structure*", so you not only have to make sure that there's no person within 500 ft, but also no structure within 500 ft.
Skimming over the wavetops, that means you've got to be sure no divers underwater, less than 500 ft deep, or no structures less than 500 ft deep. Over land, no tunnels etc.
Legally, if you were to find a structure with a 1000ft gap in it, you could even fly through this gap if you wanted to. Maybe that's the reason that they created a no-fly zone around the Millau bridge - otherwise you could legally fly below it.
Millau Viaduct - Wikipedia, the free encyclopedia