Around 3 million shipwrecks cover the ocean floors, according to the UN. Few of them contain riches. Finding them involves lengthy research and lucky breaks. Recovery can take months of work by specialist crews. Of 52 annual reports filed by publicly listed shipwreck-recovery firms since 1996, only five show a net profit. In that time Odyssey, the biggest, has had losses totaling nearly $150 million.

If profits are low, the broader costs are high. Archaeologists accuse treasure hunters of smashing wrecks while looting them. Indonesia complains that a rare Arab dhow site was destroyed in its waters: thousand-year-old ceramics, judged valueless, were thrown back in the sea.

A UN convention in 2009 banned the sale of artefacts from wrecks over 100 years old and supports their conservation. But only 42 countries have ratified it, not including Britain and America. The opposition of salvage firms shows the convention’s power, argues Ulrike Guerin, who runs its Paris-based office. Where many states sign up, as in Latin America, treasure-hunting ventures are sinking.