A serac is a large chunk of glacial ice which can be as big as a house in some cases. Seracs tend to form at the intersection of multiple crevasses, and they are notorious for being extremely unstable. If a serac collapses while people are navigating a glacier or mountain, it can cause serious damage or even death, depending on the size of the serac and the position of the climbers. Most climbers are aware of the potential danger posed by these large chunks of ice, and they are careful to avoid them whenever possible.
Often, a serac takes the form of a tall column of ice, which may have a pointed peak. Seracs may remain consistent through the course of several seasons, or they may collapse and reform frequently, depending on weather conditions and altitude. Other seracs are more chunky, and they may be liberally distributed across the face of a glacier or mountain.
The name “serac” comes from a Swiss name for a very dense, crumbly white cheese; from a distance, a field of seracs can resemble cheese, at least to an imaginative eye. The name for this cheese, by the way, is derived from the Latin for “whey.”
When navigating around a serac, climbers have to be extremely cautious. They should not anchor themselves to seracs, or use seracs to brace themselves. Even touching a serac can be dangerous, as enough pressure at the wrong point can cause the serac to crumble apart or fall. In the case of a very large serac, the results can be catastrophic; in 2008, for example, a falling serac severed a climbing line on K2, one of the highest mountains in the world, leading to the deaths of 11 climbers. Some of the climbers were trapped on the mountain by the loss of the line, while others were crushed by the falling ice. A falling serac in 1981 also killed 11 climbers, distributing car-sized chunks of ice explosively across Mount Ranier.
You can often recognize a serac, if you know what to look for. Seracs tend to stand out around the surrounding glacier or snow field, and they often look very dense and blocky, although they may be fissured with cracks and fractures which betray their instability. These hazards often form near the edges of crevasses or on steep mountain slopes, and often their peril is obvious to even the casual viewer, since a serac can look as though it is about to fall at any time.