This was in the floor of my closet. It was taped over. I studiously ignored it.
I wanted to set up my air conditioning unit in my bedroom but I didn't have any windows. When I bought my house there were no roof vents, so during the summer my house was HOT. Really really hot. I eventually got the roof vents installed (thanks, Al!) but in the meantime I needed a way to cool my bedroom while I slept. So I bought a portable air conditioning unit and figured I could maybe vent it out this hole and into the basement. I got brave and took the tape off the hole. I cautiously opened the hatch and found some sort of homemade lockbox.
I slammed the lid shut and called Z.
Me: "Hey, what are you doing? You want to come over and have lunch?"
Z: "Actually, I'm not feeling well so I'm heading home."
Me: "Okay, you HAVE to come over and help me excavate this weird box in my closet floor. There could be spiders (OH MY GOD, SPIDERS) or severed fingers or guns or cursed doubloons in there."
He came over. I have great friends.
There wasn't anything in the box, luckily, and Z helped me dismantle the thing. Now I had a straight shot into the basement.
I used the duct in the basement from the old stove downdraft that I mentioned earlier
to vent the AC unit out of the house. Thank you, former house owners, for not leaving anything scary in that box. I'm not even going to think about what you might have stored in there.