Weird Skyblock?

Okay, so a friend of mine made a new skyblock, and he told me to tpa to him to see if this was a Bug..

I don't know what happened... but this spawned on his island when he created it! I'm not sure if this is a bug or not.. or a 1 time thing.. but it is kind of odd.

Cords: X:8553.996 Y: 66 Z: 4743.717

 

Never seen it before but maybe it's like one of them jungle temples.
Was there anything in it?

 

No.. I went down to look.. it's a flat surface.. And you cant see it well in the picture.. But that one block is a Nether Brick

 

Must be some sort of chunk error.
This is highly unlikely but maybe another player built that?
I'm not really sure what that is.
Can you build on it?

 

Yeah we can build on it.. that's the weird part

 

hmm, well I don't really know what to tell you then. Might just be a spawn error or a chunk error. Doesn't seem harmful so I wouldn't really worry about it.

 

Except the fact that if you leave the chunk for 2 seconds 8000 mobs spawn on it.. (Not Literally)

 

That can't be a chunk error because the world by default should be empty. It must be player built.

 

That's what I was thinking as well, but why would something that another player built, be put onto my friends island? Without actually being added to build it.