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 -
[media]
Underwater, it looked like this -
[media]
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.
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Please provide the seed and coordinates.

Unfortunately, I have lost the coordinates.
I'm almost certain that the seed is 9210560948155018615. I will also edit the description to add this.

If you still have the world with the huge block of stone in it, it'd be good if you could upload it here.

I accidentally opened it in 1.12.2. I should have read the warning.