WTF!?!??!?!?!?!?!?!?

So I was playing on my Singelplayer world and then I was going to get wood. On my way to the forest I saw a Big hole in the ground. I've seen these in Multiplayer many times. And those in Multiplayer was just a bug, So if you walked in to them you would get stuck adn if you logged out and in they would disappear. But not this one. I logged out and in 3 times, and closed my minecraft and opened it. But nothing worked. Then I looked down the hole and saw... A billion zombies. I was like WTF!. Then I copied the wolrd and got in to Gamemode 1 (Creative) and sarted digging down. Then I came down and there was so many zombies!!!!!
I didn't do this. And it lagged like shit!
Does anybody now why this happend?
I'm actually a bit scared of playing in that world now.

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