What is this hole on the wall of my fireplace?

[Plachelli]

I discovered a very symmetrical, 1.5 inch in diameter, hole in the side wall of my pre-fab wood burning fireplace. It was covered up and filled with what looks like tin foil and cotton filling by the previous owner. If it matters, the fireplace is locwted on the 3Rd floor of a three story building. I do not know if i share a chimney with the units below me.

Questions: is this hole supposed to be there? Does it serve a purpose? Is it supposed to be sealed or left open?

Any help would be greatly appreciated. Thank you.

 

[Plachelli]

 

[Simonkenton]

It looks to me like somebody was running gas logs in that fireplace, the gas pipe went through that hole. You ought to seal it up.
Tin foil and cotton not good.

 

[madison]

Gas line as simokenton suggests. Rockwool may have been the cotton like material.

 

[Plachelli]

Thank you for the replies. Good to know it wasn't caused by damage. Is there a way to properly seal the hole? It doesn't seem like the fireplace walls are made out of brick, is there any way to determine its composition so I know what material to use to seal it?