Malfunctioning disk inside the well about 1,000 feet below the ocean floor
Excerpts from the Wall Street Journal:
The disk, part of the subsea safety infrastructure, may have ruptured during the surge of oil and gas up the well on April 20... BP officials said. ...
[P]eople familiar with BP's findings said... said much of the drilling mud may also have escaped from the well into the rock formation outside the wellbore.