Our basement has the same thing except it’s not covered up. It’s the bottom of the chimney, right in the middle of the basement. The fireplace is on the first floor behind drywall inside a dividing wall that was added between our dining room and kitchen at some point.

There’s a second smaller fireplace behind drywall on the second floor as well inside a dividing wall between our loft and another room.

The bottom of the chimney has ventilation holes and other random holes that are covered up on it.