Although hydrogen-filled balloons, such as the exploded Hindenburg (1937), had rigid structures, the flammability of hydrogen always posed a safety hazard. In 1905, two chemists discovered helium in a Kansas gas well, and this rare element was suddenly plentiful. During World War I., chemical technology extracted, stored, and shipped large quantities of helium, and helium-filled blimps in World War II. safely escorted troop and supply ships around submarines. In the 1950s, helium was useful as welding atmosphere during rocket construction and as the force needed to push the rocket's fuel to the engines.