Naturally, I again chose the scenic loop. After a short jaunt on 31W, I veered north onto the rural SR 101, which yielded lush foliage and narrower lanes. From whichever direction you approach Mammoth Cave, you're guaranteed a melange of lonely, sweeping back roads and honest-to-goodness cliches--from silos to riding stables. At this point in the trip, I started noticing small craters filled with water and ground depressions pockmarking the surrounding fields. This is what geologists call "karst" topography--landscape dotted with springs, sinkholes and openings in the bedrock. The "natural sink" depressions indicate that rainwater has entered the ground and dissolved the limestone underneath. Cave drains flush dissolved limestone away, and the surface soil settles, creating bowl-like depressions. Rain runoff collects here, and if the sinkholes become plugged with soil, a pond forms.