The world's largest cruise ship, Queen Mary 2, has returned to port in Florida because of fears it may have hit something, damaging its propellers.
The Southampton-based Cunard Lines ship, with 2,500 passengers on board, had just left Fort Lauderdale when the crew reported an unusual "shudder".
The liner is due to set sail on a 38-day trip around South America.
Cunard said the passengers were still on board and divers were about to inspect the propeller unit.
He said: "The vessel had just left Fort Lauderdale when it was discovered that there was something wrong with the unit, known as a pod.
"The passengers are all still on board and divers are going down to inspect the pod."
US coastguard said the cause of the vibrations would not be known "until we take a good look".
The QM2 is the world's largest and most expensive cruise ship. It has a gross tonnage of 150,000 tonnes and is 345 metres (1,132ft) long and 41 metres (135ft) high.
This is not the first time the liner has known trouble.
Fifteen people were killed when a gangway collapsed while the ship was in dry dock at St Nazaire, France, in 2003.
In 2004, she arrived back home from her maiden voyage three-and-a-half hours late after bow doors covering propellers failed to shut in Portugal.
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