article: And that's a conservative estimate by Unesco (the United Nations Educational, Scientific and Cultural Organization). Special Report: The Technology of Business Tech sector leading Ukraine's revival Crowd investing opens up high finance Ultra HD gives firms clearer picture Translation tech talking business Ukraine's green tech start-ups bloom They have fascinated generations of artists, writers, anthropologists and scientists. Many mark the final resting place of their crew and passengers. Some are hundreds of years old, a time capsule from distant ways of life. All are guardians of the treasures that sank along with them - be they in the form of uniquely preserved cultural heritage or cold hard cash. Excavators of the Tudor ship the Mary Rose, which sank off the English coast in 1545, found 500 pairs of shoes among the 19,000 artefacts retrieved from the site, many of which are now on display. The SS Gairsoppa on the other hand, a steam ship sunk by a German U-boat off the Irish coast in 1941, went down with over 110 tonnes of silver on board. In 2010 a US-based company called Odyssey Marine Exploration won a tender put out by the government to retrieve that silver from its resting place, 4,500m below sea level - one mile deeper than the Titanic. That sort of depth can only be reached by machine (the deepest recorded human scuba dive is currently 332m, according to Guinness World Records) and getting there is, to put it mildly, a technical challenge. "Ten years ago you couldn't possibly have done it," Andrew Craig, senior project manager onboard Odyssey Marine vessel Explorer, told the BBC. "It would have cost so much. Just getting a work class ROV (Remotely Operated Vehicle) to 5,000m - they're just not built because there's so little requirement for them." It took 3.5 hours for Explorer's onboard ROV, a 6.5-tonne vehicle named Zeus, to make each journey between the ship and the site of the wreck. With any sort of deep sea exploration, positioning is crucial. The proverbial needle in the haystack is much harder to find when it's floating in gallons of sea water, surrounded by marine life, and you can't use your fingers to feel for it. "You want to know to within 10-15 cm where things are - and you need to be able to go back to them repeatedly," says Mr Craig. In addition to Explorer's onboard sonar scanners and magnetometers (a kind of deep sea metal detector also used by the military to seek out submarines), Zeus has been equipped with an Inertial Navigation System. This captures a range of data from numerous sensors, not only to navigate a path, but also to remember where it has been. It cost over £100,000 ($157,000; €126,000) and is just one of many sensors used by the team in guiding the ROV. The technology advances still on Andrew Craig's wish list are surprisingly familiar. The first is better wireless communication - ROVs still need to send data and receive instructions by fibre-optic cable, meaning they remain tethered to the ship. The second is
summary: Far beneath the ocean waves, nestling silently on cold dark sea beds around the world, lie the remains of about three million shipwrecks.