What is this? (Photo inc)

[welliwasntexpectingthat]

It is where you'd expect a little hearth to be in front of a fire in upstairs bedroom. Except it is domed in shape and doesn't lift out like hearts usually do.

Wild guess, but has a previous owner lifted a hearth slab/tiles and tried to repair the mess and it turned out badly done?

I just don't know. The dome shape is odd though...

What is it made of OP ? It looks like a slate hearthstone underneath, with the domed area in concrete ?
Could have been a base for a gas fire maybe, if so.

I'd be concerned about it being a Portal to the Nether Realms if if was on the ground floor, but since you are upstairs and you don't mention any weird runes or discouraging notes then I say hit it with a cold chisel along the join and I bet it'll lift right off.

As for why...who knows? How have you not all been tripping over it?