What is this vertical metal pipe in my backyard? old well? septic?
Since the pole is so thick and there's only one of them, it could have been the base of a satellite dish - like one of the old 8' dishes (big, not sure of exact measurements).



The height is about right for that, and it's kinda short for a clothesline.

It filling with water is just from the rain.  I've never seen anything plumbing or well related that looks like that.