Allen said that in preparation for the storm, crews had pulled the drill string out, put a plug known as a "packer" in the casing pipe and filled the riser with seawater so the pipe would be stable.

CNN: STORY HIGHLIGHTS

As early as five hours before the explosion drillers noticed unexpected loss of fluid in the riser pipe, indicating leaks in a part of the blowout preventer.

FORBES: Top Kill: BP's Must See TV

Inside the riser pipe is a space about half a meter in diameter, large enough to hold the spinning drill pipe and bit and to allow the return flow of drilling fluid.

FORBES: The delicate art of sucking up