How to recognize a hole in the ground

I saw a suspicious area on the edge of a cornfield. I got out of my car to look, and immediately realized it was a sinkhole, even before I saw it. How did I recognize it? First, I saw the flies. I don't understand that, because once I was battered by the stench, I could not understand how I smelled nothing as I got out of the car. This might be a good sinkhole to use in my investigation of ground-water flow in this area, but there is something aesthetically unappealing about pumping water into a sinkhole with a ripe dead cow barely covered over in it. I guess many people still do not understand where their drinking water comes from (on-site wells in this area) and find it easiest to dispose of trash and dead animals in ready-made holes. This seems to be standard practice in many karst areas. This farmer is lucky--his house is upgradient. His downgradient neighbors are not as lucky.