Combining power and agility in one sport, climbing requires strength and flexibility to ascend steep rock wall formations. Both a physical and mental game, it can offer tough and dangerous physical challenges. Sport climbing involves the use of securing to bolts for protection. Main disciplines include sport climbing where the climber uses fixed protection, alpine climbing where the climbers place protection as they ascend multiple pitches to the top of a peak, and bouldering where powerful, dynamic moves are practiced on short climbs typically without the use of a rope. Boulder and climbing gyms offer a similar training experience in an indoor environment.