A theory based upon the "waterhammer effect" is developed to explain the cause of the blowout of the Key 1–11 well on October 4,1981, operated by Apache Corporation in Wheeler County, Texas. The real cause of the accident was not known. The operating pressures before blowout were not sufficient to stretch ″ casing 80 feet and to cause this accident. However, this analysis shows that high pressures generated due to waterhammer appear to be the probable cause. It was assumed that the tubing suddenly ruptured (possibly from corrosion), causing a pressure wave to develop in the KCl water in the annulus between the ″ tubing and ″ casing. The upward moving wave reflected off the wellhead and moved downward. Depending upon the amount of gas on top of the water in the annulus, pressures up to 30,000 psi were computed. Pressures of these magnitudes were shown to stretch the ″ casing in excess of 80 feet which correlates with the evidence at the blow-out site.