A grate in a sidewalk had an odd metal ring spot-welded in one corner. There was not a valve under it, so its purpose remained unknown. Naturally that ring formed the letter O.

Part of the brick sidewalk had been cut into this letter O. Evidently the larger portion surrounding it was an access hatch. This cut had been made to create easier access to whatever was underneath.

Another hole in a brick sidewalk, but this time the removed plug both fell apart and was replaced upside down. Again the cut formed the letter O.

An odd round barrel top had been used to form a gate in a stone wall. The gate didn't appear to open, but a hole had been chewed into its base. Note the valve in the lower left of the gate as you face it. The gate itself was the letter O.