When skiers turn their way down a slope, the ruts they cut outline bumps called moguls. Once the mogul has begun to raise its head, subsequent skiers tend to follow similar paths around it, deepening the grooves and hence raising the bump. Ski resort operators take two opposing tacks with these human-caused landforms. On some slopes they groom them flat each night to make the skiing easier; on others they allow them to grow, in hopes of attracting the most intrepid downhillers.

Bill McKibben