When the heavy drilling mud that provides the pressure needed to keep things from coming up the well was removed, first as a test, then as part of a procedure for closing down the well and moving the rig, telltale signs that something was wrong were missed.

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He said in such a situation, normal procedure would be for the contractor's crew to secure the well by closing off the huge valve sitting on the seabed known as a blowout preventer.

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