At low tide you can drive on the bottom of the North Sea shores of the Wadden Sea between Jutland and the island Mandø. At high tide there would be around 1 meters of sea water here.
This is the normal road, the Laaningvejen. Just to the south is also a road but it's more risky to drive on.
On this video you can see a car that was caucht at beginning high tide so it had to be pulled back to safety by the Mandøæ-Bus:
http://www.youtube.com/watch?v=53epgCaZ400