I was flying around the ocean in creative mode, looking for some underwater ravines, trenches. Then I found something strange.

From the top, I saw this -

Underwater, it looked like this -

It's probably not a chunk error. It is made of different kinds of blocks, such as stone and gravel. Also, it is in just a normal ocean biome, not a stone beach or something like that.

I lost the coordinates, but the seed is 9210560948155018615. Not sure if this will be any help, though.