A pothole in a streambed, often surprising for its perfect roundness, was formed and enlarged by the grinding effect of pebbles and cobbles swirled around by the water’s circular current. Scour hole is a regional variant. Prairie pothole refers to a large depressions in the hummocky terrain of a glaciated plain. As a general term pothole may be most familiar to people living in the pothole country of the northern Plains, a landscape of natural ponds scattered far and wide, habitat upon which migratory waterfowl, especially, are dependent. It’s these potholes that gave rise to Minnesota’s moniker, “The Land of Ten Thousand Lakes.” But hikers and river runners from Georgia to Oregon know potholes as characteristic of stream ﬂows over certain types of substrates—basalt, for example. And that during low water or after a ﬂood these dry scour holes often contain small treasures neither wind nor water has swept away, like polished rocks and the delicate exoskeletons of insects. When speleologists talk of potholing they are using the term metaphorically to describe vertical cave systems.