A plane flying from Newark to Rome returned safely Thursday to Newark Liberty International Airport after dropping from 37,000 feet to less than 9,000 in eight minutes.

According to the Federal Aviation Administration, the crew reported a pressurization issue on the plane. The Boeing 777 was headed to Rome–Fiumicino International Airport when United Flight 510 was forced to turn back around an hour into the flight. Passengers were flown to Rome on another plane.

The FAA says pilots are trained to make controlled descents to below 12,000 feet if there’s a pressurization issue because below that altitude does not require supplemental oxygen.

Tato Lovere, a Jersey City woman who booked the trip to celebrate her birthday, told News 12 New Jersey the uncertainty of what was happening was the worst part. Lovere said her daughter tracked her flight and saw the plane heading in circles above Canada.

The FAA is investigating possible air pressure problems in the cabin.