If you're the type to get nervous during flight turbulence, then the following story might outright terrify you.

United Airlines Flight 1676 encountered extreme turbulence on its way from Denver to Billings, Montana. So extreme, several passengers were literally thrown out of their seats. Three crew members and several passengers were injured and a flight attendant remains in critical condition.

USA Today reports one passenger's experience.

"I thought, 'Wow! Yeah, this is it," said Keri Mullins. She was among the many passengers caught off guard when suddenly swerved right and plunged. "Everything flew out of everyone's hands," she recounted. "It was quite surreal."

The turbulence hit the plane so fast that even the crew seemed unaware of what was happening. Passengers screamed and parents called out for their children. Passenger Bill Dahlin told KTVQ that one woman flew into the ceiling so hard it cracked the panel.

United Airlines has not reported yet on what may have caused such a severe situation. They say their flight safety team will investigate what happened but their current focus is on assisting anyone who was injured.

Weather.com meteorologist Alan Raymond has a theory.

"At the time of the incident, skies over the Intermountain West were partly cloudy but winds were howling over southern Montana and northern Wyoming," Raymond says. "The plane likely encountered what's known as 'clear air turbulence' which is nearly imperceptible to the aircraft's onboard radar."

Clear air turbulence is commonly referred to as "air pockets."

There have been other instances of injuries due to clear air turbulence. In the case of United Airlines Flight 826, it even resulted in a passenger's death. The Boeing 747-100 was hit by severe turbulence leaving Tokyo on Dec. 28, 1997, injuring 15 passengers and forcing it to turn back. One woman later died after landing.