Today while mowing a lawn we spoted a large 5' round mound in the lawn. When I walked over to it and pressed on it, it shook like jelly. I stood on it and it was like I was standing on a pitchers mound. I got my rake out and used the handle to puncture the turf and release the water. No irrigation heads are there and it's not on a septic system. Anyone have other suggestions on what will cause this build-up of water under the turf?