Costa Concordia: Cruise ship survivors found

Shipping experts say they are puzzled as to why such a modern and well-equipped vessel should have run aground in well-charted waters.

The £300m ($460m) ship had been travelling in calm seas, along a familiar route.

How it came to deviate from its course by 3-4 nautical miles - and why it capsized so quickly - will be central for investigators trying to establish the cause of the accident.

The investigation will focus on why a modern ship, with the latest safety equipment, travelling on the same route it travels 52 times every year, seemingly veered off course and hit what the cruise company has described as "a big rock".

There are a large number of possibilities ranging from human error, to technical failure, to a combination of the two.

It is possible the crew simply made a mistake and steered off course. They may have been misled by faulty navigation equipment.

Other reports suggest there may have been some kind of power failure.

These big ships rely on their electric generators for steering, power, lighting and so on. But that is why they have back-up systems that kick-in in case of emergencies. Recorder is taken from capsized ship The "black box" recorder has been taken from the capsized ship

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