In May 1952, BOAC started passenger jet flights with De Havilland Comets. All Comet Is were grounded in April 1954 after three BOAC Comets exploded in the air. Investigators discovered serious flaws in the plane's structure. These flaws were called metal fatigue, which occurs because of the constant pressurization and depressurization of airplanes. De Havilland engineers then designed an improved Comet, which was called the Series 4. In 1958, BOAC used the new Comets to become the first airline to fly jet passenger services across the Atlantic. American companies also built successful jet transports in the late 1950s and these aircraft quickly dominated international air transportation. The most successful was the Boeing 707, which began services across the Atlantic and across the United States in 1959.