I was visiting a field one time and brought out my two metal skewers to hold the tail of the plane and one of the gentlemen there strolled up next to me and kindly explained that the field had an irrigation system and one of the lines ran right through the pits. They explained where it was and that I should not stick anything in the ground around that area. Maybe there was an irrigation system and they've had too many problems with "unauthorized new sprinklers" popping up.