(CNN) Severe turbulence rocked a Houston-to-London flight over the Atlantic, forcing its pilots to make an emergency landing early Wednesday in Ireland and sending 12 people to a hospital -- a flight one passenger called the most disturbing he'd taken in decades.
United Airlines Flight 880 landed unscheduled at western Ireland's Shannon Airport shortly before 6 a.m. local time so that people could receive medical treatment after "severe and unexpected turbulence," the airline said.
The shaking began overnight about halfway over the Atlantic while many passengers were asleep, followed by "very, very severe drops in altitude," passenger Gregory Giagnocavo told CNN after landing in Ireland.
"It was the most frightening and disturbing flight I've been on in 30 years," Giagnocavo said.
The Boeing 767-300 was carrying 207 passengers and 13 crew members. After it landed, 10 passengers and two flight attendants were taken to a hospital, United said.
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Three children were among them, University Hospital Limerick said. The 12 were treated primarily for soft tissue injuries, minor head injuries and lacerations, the hospital said.
By late Wednesday morning, only a flight attendant remained in the hospital, according to the airline. The plane resumed its flight to Heathrow Airport at 12:11 p.m. (7:11 a.m. ET).
Giagnocavo said the scene on the plane was "pure chaos" before the emergency landing.
"(A) flight attendant was cut on the side of her head, and blood was running down her arm," he said. "Babies crying -- and quite a few people very shook up. Loose things seem to be everywhere.
"Fortunately it occurred when most people were sleeping, so most people were in their seats with seat belts on."
Giagnocavo put the number of steep drops at four; another passenger, Nikki Hartin Boriac, said it was two.
"(The first one) was a couple seconds, because it was long enough to think about it. While you're dropping, everything is flying up, so anybody who wasn't buckled flew up," she told CNN affiliate KTRK-TV in Houston. "And then we had a few bumps for a second, and then there was another really big drop."
The flight attendant across from her "thought she was buckled, but she wasn't and she hit her head really bad," Hartin Boriac said.
But flight attendants did well to keep people calm in the aftermath, she said.
United "is providing care and support to customers and crew of Flight UA880," airline spokeswoman Erin Benson said.
The cause of the turbulence wasn't immediately known, the airline said.
But CNN senior meteorologist Brandon Miller said the jet stream could be to blame.
"It appears that the flight path took the flight into what we call a 'jet max,' or the fastest part of the jet stream," Miller said. "This is an area which will often see 'clear-air' turbulence, as the wind speeds change rapidly with height. It is hard to avoid as well, as it would cost a lot of time and fuel to fly around the jet max, and you cannot see the turbulence -- hence the name 'clear-air,'" he said.