The leak was attributed to the accidental pouring of water into the tank.
Water reacts very vigorously with MIC, causing heating of the liquid. In Bhopal, the mix- ing of water with MIC increased the temperature of the liquid in the tank to an estimated 400°F. The high temperature caused the MIC to vaporize, leading to a build-up of high pressure within the tank. When the internal pressure became high enough, a pressure-relief valve popped open, leaking MIC vapors into the air.