Grass Shape in the Yard

I make a point to call and visit my parents once a while.  I like making sure that they're still kicking it.  Anyways, my parents have this huge yard with a vegetable garden and still have room for this unusual thing growing in the grass.  That only a small portion of the lawn is a bit greener than the rest and in a shape of a circle. I'm curious as to how this happened so from talking to dad, his reason was that the fertilizer didn't get washed away and has collected itself randomly in a circle.  Sure that would make more sense if I'd checked to see if the ground inclines or not.  Hmm ... but a little part of me suspects aliens :)