The De Havilland Comet, the world's first commercial jet airliner, began service with BOAC (British Overseas Airways Corporation) on May 2, 1952. It was a groundbreaking aircraft, offering faster travel times and greater comfort than previous propeller-driven planes. However, on January 10, 1954, a Comet aircraft, registered G-ALYP, disintegrated in midair while flying from Rome Ciampino to London, crashing at 27,000 feet and killing all 35 people on board. The investigation later found that the disaster was not caused by the plane’s square windows, as initially suspected, but by a fatigue crack that started near a rivet hole on the roof. The crack expanded with each pressurization cycle, eventually causing the fuselage to tear open. The Royal Navy recovered much of the wreckage, and the Royal Aircraft Establishment in Farnborough, led by Sir Arnold Hall, reconstructed about two-thirds of the G-ALYP for analysis. A second Comet, G-ALYU, suffered a similar fate on April 8, 1954, when it broke apart near Naples, killing 21 people. To understand why the planes were failing, Hall conducted extensive testing by immersing a complete Comet fuselage in a large water tank measuring 100 feet by 20 by 16 feet (approximately 30 meters long). On June 24, 1954, after simulating 3,057 pressurization cycles—equivalent to 1,221 real flights and 1,836 simulated—the fuselage split open, beginning at the corner of the front emergency hatch. Previous testing had shown that parts of the Comet could withstand up to 18,000 cycles, but these tests were on sections of the fuselage, not a complete aircraft with all its windows. Dr. Walker, a structures officer at the Royal Aircraft Establishment, noted that a 3-to-1 difference in performance between test sections and the full fuselage was not unexpected, as variations in material strength and stress distribution can lead to such discrepancies. The cracks in the G-ALYP were found to be concentrated near two navigation antenna openings, where opaque fiberglass panels had been used instead of glass. On August 12, 1954, a section of the fuselage arrived at Farnborough and clearly showed a fatigue crack. A commission led by Lord Cohen, established in October 1954, concluded that the manufacturing process had worsened the issue. Reinforcements were riveted instead of glued as originally planned, and the riveting process left imperfect holes that were prone to cracking. While the square windows did not directly cause the failures, their sharp corners concentrated more stress on the fuselage than expected. In the tank test, it was the emergency hatch, not the antenna, that gave way. An Italian trawler recovered a cabin window, which may have been the primary rupture point, but the investigation was not reopened due to a lack of conclusive evidence. The Comet disaster led to major changes in aircraft design, including the use of circumferential stiffeners to prevent cracks from spreading and the adoption of oval or rounded windows in all modern commercial airplanes to reduce stress concentrations. The piece of the G-ALYP roof, which includes the two antenna windows, is now displayed at the Science Museum in London as a reminder of the importance of rigorous engineering and testing in aviation.