Rat,
If he/she from just about anywhere in Canada (i take this from his handle) you pretty well have to roll your lawn in the spring time from the frost heaving, making it very uneven in places. Its a ritual for me every spring at home and the cottage, if not you can beat yourself to death when you start mowing. If you have any rocks, stumps or anything embedded in the ground the frost will push it out in the spring. My neighbor at my cottage had a railroad tie for a post under his deck with five feet of it in the ground and the frost pushed it up 8" last year jamming the the front door closed and breaking a side window in the cottage. Mother nature can destroy anything we can build one way or another.
Steve