BBC News Published image captionA skier watches the slow moving avalanche A group of extreme skiers have had an unusual encounter with a "slow motion" avalanche in a gully on Ben Nevis. Avalanches are usually "explosive" events involving a mass of snow sliding suddenly and rapidly down a mountainside. The group was on Ben Nevis' north face on Saturday to ski its steep descents. They were surprised to find the avalanche moving slowly down Observatory Gully from the summit of Britain's highest mountain. Al Todd, who was part of the group, said late spring was usually the safest time of year for tackling the north face due to the snow conditions, and reduced chance of avalanche activity.