It's a sprinkler head.

http://www.krain.com/index.php

I have no idea how you could turn it on though. Do you have a basement maybe with a valve that goes outside? Garage?

k,

I agree with extreme. If you can not find the controller or valve box that turns it on, the the only way find it is dig up the sprinkler head and see which way the lateral pipeline is going. You can continue to find it by digging holes spaced along its route till you get to the valve box or buried automatic valve controlling it, then on to the controller itself. The system could have been removed but the sprinkler never removed. You will find out soon enough by digging holes. Let us know what happens. GL JMHO Aloha