During World War I, the U.S. Army Air Service watched and photographed enemy troops from balloon baskets. Each balloon held a vast volume of flammable hydrogen gas that could fill almost half of an Olympic-sized swimming pool. Observers needed to be able to bail-out if they were hit with enemy aircraft fire and set alight. Parachute containers hung outside the balloon basket. Observers wore harnesses attached by lines to the parachute inside the containers. When observers bailed out, the lines pulled the parachute open.