I was travelling in a minecart over a railroad bridge. A section of the bridge was missing and I fell into an ocean.
The content of a 16x16 square has disappeared, and replaced by an 'ocean-chunk'. Now there is a 16x16 hole in the hill, with water inside. This water is complete with sand bottom.
-This part of the save was quite old, 2 years maybe, this particular chunk loaded correctly many times.
-The biome name is 'ocean' wherever you go in this part.
-I only played survival mode, normal difficulty.

My worry is if more valuable parts of my world will disappear as well, that would spoil it all.