When the Deepwater Horizon oil rig suffered a catastrophic explosion and blowout on April 21, 2010, leading to the worst oil spill in the history of the petroleum industry, the well’s operators thought they would be able to block the leak within a few weeks. On May 9 they succeeded in lowering a 125-ton containment dome over the broken wellhead. If that measure had worked, it would have funneled the leaking oil into a pipe that carried it to a tanker ship above, thus preventing the ongoing leakage that made the spill so devastating. Why didn’t the containment work as expected?