Officials discovered that gas hydrates, ice-like crystals lighter than water, had built up inside the 100-ton metal container. The hydrates threatened to make the dome buoyant, and they also plugged up the top of the dome, preventing it from being effective.

“I wouldn’t say it has failed yet,” Doug Suttles, BP’s chief operating officer, said at a news conference in Robert, La. “What we attempted to do last night hasn’t worked.”

As a consequence, crews had to lift the dome off the well and place it on the seabed.