The things you don't see

I have walked past this, whatever it is, hundreds and hundreds of times on my morning walks and I have never noticed it. How does that happen! The only reason I have now noticed it is because my better half told me that she treats "the green thing" as her turn-around point for her walks. (We walk beside a golf course, right!!) Anyway, the joke's on me because I have never noticed it.

We think it has something to do with the water and sewerage system. Any clues?