[CaptainHaplo]

In the final stages of closing the well, a concrete "plug" is poured to seal the well. Farther up, another concrete plug is poured to insure its clamped shut. Haliburton was responsible for the concrete pours. After the first one, they noted some problems in writing to both Transocean and BP because the testing indicated there could be some gas still leaking through. This is why the mud cycle should have run for 8-12 hours minimum (to get the mud at the bottom of the pipe up to be checked for gas absorbtion) and that BP only circulated for 30 minutes - not enough by a mile. Had they done so, they would have known they had a gas leak, and it was unsafe to swap the mud out for seawater (which allowed the gas up the pipe and led to the explosion. Normally the mud isn't swapped until the second plug is in place and tested, but BP did get a permit to only use the one, and ignored the warnings that there was a problem.