On our way to Dieppe, Maggie and I spent a few days on the West Coast of France. It's mostly very flat, intricate byways amongst salt-marshes. Easy riding but enjoyable. One of the rides we did was to cross the Passage du Gois, a road that joins the island of Noirmoutier to the mainland.
It's a very ancient road, not surfaced until the 1930s, and it is only passable for about three hours each tide. The rest of the time it's under the sea - about ten feet under at high tide. There are towers you can climb to wait if you misjudge it, but it would be quite a long wait, and your car wouldn't be much use afterwards.
From a cycling point of view, the interest is that the Tour de France starts there on the 2nd July. It has crossed the Gois once before, in 1999, when the Swiss favourite Alex Zülle crashed on the slippery Gois and lost forty seconds. He never recovered this time, and came second to a young American called Armstrong, taking his first Tour win. The rest, as they say, is history.