I damaged something with my lawn mower, but I don't know what it was, so I have no idea how to go about replacing it. I'm 90% sure that the "something" was part of my irrigation system. Please let me explain:

In the corner of my back yard, there is a round, white plastic thing that sticks up about an inch above the ground level. It is located at the low point of the property, at the bottom of a slope.

In any case, I tore it to shreds with my lawn mower. After breaking it, I could see water inside it.

I've taken pictures and linked to them below in hopes that someone can help me identify this item and obtain a replacement. (In one of the photos, water can be seen inside.) Any advice (What it is, where to get another, installation caveats) would be greatly appreciated!

http://img848.imageshack.us/img848/5391/dsc00567j.jpg

http://img695.imageshack.us/img695/6849/dsc00568cp.jpg