You need fine sand and lots of water so beaches weren’t exactly wrong but it’s still somewhat rare. Warning sign on Texel (Netherlands). Swamps are another candidate though there’s also other traps there, as well as mudflats… or at least it’s a very similar phenomenon the German term is different (Schlickloch vs. Treibsand) but it’s essentially the same thing. Don’t go walking from island to island without someone who can read the ground, maps would be useless they change every low tide. Also don’t leave when the water is already coming back. Also, don’t complain to your hotel that the sea is gone it’s a feature, not a bug.