An ocean biome chunk cutting into land in other biomes causing them to look like this. This is a freshly created 1.4.2 world.
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Chunk error, by exploring old and new terrain or by creating a new world with the same name as an old world.

How old is original explored land? There is point where generator was changed, but not for while.

This is in fact a new world that i generated from The Halloween update. I did not run it on any other versions

What is the seed ?

How do i check the seed? 😛

It's /seed unless its server then you can't see the seed.

Duplicate of MC-315.