For those who've always wondered what's behind a closed fireplace, here is a peek into one possibility.
For us, it was the exhaust for our old boiler and some other earlier conduit (large pipe in center through hearth) whose function we are not sure of. As you can see, it created a lot of damage to the hearth and firebox.
Basically, the parts hearth and firebox had been removed to make room for the stainless steel vent and the other pipe.
Once the vent pipes were removed, there was a hole through the hearth that extended through the sub hearth down into the chimney base in the basement. Our mason Edward had to build this back up so that he could repair the hearth floor.
He used a collection of field stones...
That he mortared into that space to establish the sub hearth.
Edward was able to locate a source of 200 year-old local bricks to complete the repairs.
As Mike said, it looks like it's been there forever.