On May 26th BP, the company responsible for the well that Deepwater Horizon had drilled, started to pumpheavy drilling mud into the well's blowout preventer, a set of valves on the sea floor.
This is what this data is supposed to monitor, but the well flow evidence would appear to have been ignored, because at this point the BP rig supervisors should have gone to a well kill operation and started to pumpheavy mud back into the well bore to restore the primary control mechanism.