I'm at Primal Giants and I fell THROUGH the ground and into an ocean of water beneath it. I had to use a traveler's map to Keplan to escape the situation.

You can also use esc-esc to get out.

Can happen anywhere for various reasons, all of which amount to locations or situations where the game has problems putting you on the surface, so you end up under the map.  Depending on where it occurs you could end up falling a distance and dying. I haven't been quick enough to think to try to dive in that case, maybe next time.