Free solo climbing, also called free soloing, is when a climber(free soloisst) climbs alone without the use of any protective gear. They don't use any pads or spotter, so if they fall, they fall to the ground. They reley on their ability to get them to the top, and to keep them from falling. The free soloist usually climbs above safe heights. That means that a fall could lead to serious injury or death.
