A questioner said:
What is this hole we found under the carpet?

Some of the answers:

1. The way its nailed in place I’d say duct work more than plumbing related, probably a no longer used heating duct.
2. I  have the same hole. What is below it? Mine went to the kitchen where a central heater was vented up through the floor.
3. I still have some of those same size ducting on my forced air furnace in my home. Usually there is a register cover over top but if it is abandoned they may have just been lazy and covered it over instead of removing it.
4. In my old house we had one that was still active and they just carpeted over it for no apparent reason.my elderly cat liked to lay there in the winter because it was soft and warm.
5. Those can also be for AC return vents around the house. If that’s the case, there was once a wall with a duck [edit DUCT] to bring the air up to an air conditioner.It could be that during a remodel, that system was abandoned.

   A wider shot so we can see more of the floor around it to look for signs of old walls would be helpful.

6. Looks like it used to be a toe kick. Was there a cabinet there once?
7. Maybe an air intake for a wood stove
8. Absolutely a combustion air inlet. You cant have something burning fuel without sufficient air moving into the home to make it work, so probably and old stove or something was there.
9. It’s an air duct for a floor register
10. Old house, could straight out be a duct that goes thru. My dad’s old house has remnants of the original round ductwork from the wood stove still inside the walls & floors, covered up now.
11. The thin metal indicates it’s not a drain or pipe for water. It’s most likely an old heating duct.
12. Alright. I scrolled and scrolled and read countless theories and little semantic asides and quite a few seemingly unrelated stories as well but no one, not one of y’all, is going to address the bit about the bricked up room below this?!? You have a mystery room sealed off from the rest of the home and you haven’t done even a little exploration?!? I’d have been down there with the inspector during the sale and again with a sledgehammer the moment I got the keys.
13. It’s rented so unfortunately no sledgehammer moments – as much as I’m tempted! We did ask the landlord about it when we first looked around the basement but they just said it was already sealed up when they bought it and it’s mostly just filled with rubble. Still, why seal it off…
14. Wait, so its an entire room in the basement just totally sealed off?? And the landlord didn’t open it up to check inside it when they bought the house? Also, they said it was already sealed at purchase, so how do they know its “mostly just” rubble? Unless they opened and then resealed it? Also why use “mostly”, it’s either all rubble or there’s something else in there with it
15. It might not be a full room then. Seen a few that are half height, or part size for one reason or another and because it’s not a real room, they were a cheap and easy place to throw building rubble when being built. It’s not uncommon. There’s no reason to leave a void like that open (because it’s not clean / finished / very usable, and more random space open = more space to heat and resources to make neat). Kind of like the eaves of a house, and why those are often squared off from a finished attic.
16. I giant hole like that covered with just carpet, and no one knew until now? No one has ever stepped on that spot before?
17. Yes, this is for ventilation of the space between your floor and the ground or foundations to prevent damp and rot of wooden beams, which I assume there are due to the floor being wooden boards. We have these in my house except that they have decorative brass covers.
18. Had the same thing in my house. Was a dryer vent that went into the basement and I turned it into a beer can/bottle chute. Just put a lined garbage can underneath, worked like a charm and made recycling so easy.
19. We had this at home when I was a kid. The one and only wood-stove of the house was just underneath in the room below so heat would come up on the first floor.
20. Based on your comments I’m guessing there was some sort of furnace down there at one point and this was where a duct from it went through the floor and up to the next level (is there another floor above this?). The fact that it is round at the floor level implies it once connected to another section of ducting at some point. If it were rectangular, it would have likely been the termination point for a floor register/vent.
21. Assuming at one point it wasn’t covered by carpet, but under a cabinet, it could be what’s called a “toe kick” heat vent.You then cut a small opening low in the cabinet and cover it with a louvered grill.

    It’s commonly found where I am in the US under a kitchen or bathroom cabinet by an outside wall, where you cant put a heat vent in the floor since it would get stepped on.

22. If it’s in an area where there was a cabinet, it may be an air duct that ended in a rectangular metal register with a grill where the cabinet toe kick would be. I have a vent just like that in my kitchen (installed 1954).
23. Please update with what you find underneath!!!! I just bought a house built in 1937, I am determined to find something different and fun. No luck yet but I have time!
24. In the days of gravity heat, many times they just put a duct or opening in the floor/ceiling to allow warm air to rise to the upper floor. No duct required in those days.
25. its a old well. most houses built long ago had wells for drinking water but it looks like it was built over and covered by that rug.
26. Air intake for a fireplace? Is the wall at the top of the picture an outside wall? See if there’s signs of something being bricked over or a metal fixture out there.