Potential Answers

Trevors-Axiom- said:

It looks like an in-ground dog poo compost bin, but I can’t find an exact match. Does the top come off?

MoreTatersPlz said:

Depending on its location in your yard and the water/sewer line setup, it could be a sewer cleanout or a shutoff valve for the main city water line (if you don’t have a private well). In our last home, the shutoff line was in the front yard and the sewer cleanout in the backyard.

Post in r/whatisthisthing:

I recently bought a house and found this in the backyard. It looks like a cap, about 6 inches in diameter, made of metal and embedded in the ground.

Saltyhumor said:

This likely relates to water—rainwater, freshwater, or wastewater. Are you on a well and septic system or city water/sewer? How close is it to the house? Could it be related to a sump pump? Are you on a slab, crawlspace, or basement?

Anonymous22006 said:

My guess is an old chain link fence post that rusted off at the ground. Instead of digging it out, someone simply put a post cap on it.

Silly_rabbi said:

It looks like a cap for a vent/access pipe leading to drainage from the house. My mom has one in her yard for workers to access the drainage system and clear blockages.

GabeLade said:

My first thought was that it’s a cap for a metal post used in chain link fence construction.

Meathead-12 said:

In my neighborhood, we had umbrella-style clotheslines stored in the garage when not used. A metal pipe was placed in the ground about a foot deep, with a cap on it when not in use.

AugustWestGarcia said:

It could be the breathing cap on an old underground oil tank. Check with your Townhall for records of any abandoned tanks on your property.