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Last Updated: Saturday, 19 May 2007, 14:05 GMT 15:05 UK
How to find a treasure trove
Ground crew unload coins recovered from a shipwreck
17 tonnes of coins were recovered and taken to the United States

A 17th Century shipwreck with a fortune in gold, silver and jewels has been found in the Atlantic Ocean, most probably off the Cornish coast.

The haul made by treasure hunters from an American company called Odyssey Marine Exploration is likely to make more than $500m (253m).

So just how did the explorers find one of the most lucrative shipwreck sites of all time?

First things first, this venture had nothing to do with luck.

It took months of painstaking research, dozens of dedicated crew members, a needle-in-a-haystack type search and several millions pounds to recover one of the largest coin collections ever salvaged.

An almighty task, but to those in the world of shipwrecks, pirates and treasure troves, this was their El Dorado.

Seabed scan

Shipwreck expert Richard Larn, of Shipwrecks UK, told BBC News the explorers would have probably spent an entire year researching the ship to find out exactly what it carried, to whom it belonged and to whom the cargo belonged.

They would also consider how accessible a ship which sank "seven leagues under the sea" near Land's End could be.

"They would then draw a huge circle around the spot and double it, and then start 'mowing the lawn'," he said.

This involves travelling up and down a 100-or-so mile channel, turning around and then moving several yards across before heading back.

Odyssey's Remotely Operated Vehicle
The sister ship uses an underwater robot to salvage the coins

In the meantime, a multi-beam side scan sonar would bounce signals down to the seabed to show signs of lumps which could indicate a shipwreck.

A magnetometer would be towed by the ship to detect any anomalies in the usually straight lines of the earth's magnetic field which can be diverted by ferrous metals that can be found in cannons or shipwrecks.

Behind the leading ship would be sister ship, called into action if there is a hit. It would then take co-ordinates and lower an underwater robot into the sea in a bid to salvage treasure.

Mr Larn said at 800ft down, the wreck was not beyond diving, but beyond "economic diving".

Finders keepers

The cost of the whole operation is likely to have run into millions.

Odyssey have not put a figure on their costs, but Mr Larn suggested a two-year search, a crew of 80 and the running costs of two huge ships could have been at least 50,000 a day.

As a commercial company, there are huge costs to recoup.

So while it is mainly a case of "finders keepers", Odyssey needed to be sure that as much of the haul as possible would remain theirs.

Odyssey co-founder Greg Stemm, left, and project manager Tom Dettweiler examining a coin
The Odyssey team are to return to the shipwreck

Critically, the coins were taken to the US. Had they been brought into the UK, the Receiver of Wreck would have impounded them until ownership was decided which could take years.

Lawyers will have been alerted to deal with any claims to ownership.

As some experts believe the wreck to be the Merchant Royal, an English ship carrying stolen Spanish treasure which sank in 1641, claims could be made by Spain or the British government.

And what of the ship and its treasure?

Odyssey has been very secretive about the recovery, saying it is important to limit speculation about the wreck's identity and the value of the haul.

But on its website it says more than $33m was made from another recovered shipwreck, the SS Republic, lost in 1865 off the US coast.

It says the coins from this latest wreck will be sold off to collectors, with a sample of the different varieties of coins sold for study and display purposes.

And there may still be more to come.

The site says another major excavation may reveal more coins and a "large number" or artefacts.

But in the meantime, is there a danger that others may raid the treasure trove?

Mr Larn thought it unlikely. "It's in very deep waters. You won't find it easily," he warned any prospective bounty hunters.

Record wreck 'found off Cornwall'
19 May 07 |  Cornwall
Shipwreck yields record treasures
18 May 07 |  Americas
US shipwreck could yield millions
18 Aug 03 |  Americas

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