power lines
We always did a survey of the area above 500' first to see where they were.
The first few runs of the area we started high and worked down lower and lower each time. Also, we established a minium safe altitude to clear any obsticles in the area, if you got lost or forgot where in the paddock you where, you'd climb to that hight and figure it out.
Low level is great flying... but you gotta be safe when you do it!