In North America, the easiest ski runs are marked by green circles, and are typically fairly flat and smooth. Sometimes known as "bunny slopes", they are usually groomed by specially equipped snowcats every night. A blue square marks slopes of medium difficulty; these blue squares may be steeper or narrower than green circles, or they may be left in a natural state rather than machine-groomed. A black diamond run is yet steeper than a blue square and often involves challenging terrain such as moguls, narrow passes, unmarked obstacles, double fall lines, or gladed sections. A double black diamond is for experts only; these trails are steep, rarely groomed, and often left in a completely natural state. There is no standard for these designations, however, and each ski resort determines them relative to their own terrain difficulty.
