A Mysterious Black Hole
You know how in horror films they always have to check every dark corner of the house, they always have to go down to the basement and follow the mysterious sounds, and they always stuck their faces in mysterious, dark holes? Well, this person was curious about this hole in their house, but they turned to the internet for answers.
Luckily, people of forums such as r/whatisthisthing have all the answers. Someone pointed out this wasn’t a formidable peephole for the neighbors, but rather a fixture for a chimney. Chances are there used to be a wood stove near this wall, and a chimney was fixed to the roof.