The idea that there are wrecks full of treasure, sitting on the ocean floor waiting to be salvaged, might seem fanciful, but it’s entirely true. And it is estimated that there is around $60 billion of unclaimed treasure still out there.

UNESCO has said that there are as many as three million shipwrecks at the bottom of the world’s oceans. Roughly 3,000 of them are thought to have contents still onboard that are valuable enough to make a salvage mission worthwhile.

For centuries, ships have sunk while transporting precious metals and gems over vast distances. From the Spanish and British Empires bringing back ill-gotten gains to the motherland, right up to ships carrying bouillon for payment in WWII, there are huge wrecks that haven’t been touched since the day they sank. If you have the initial time and money to start hunting, the rewards can be huge.

Of course, it helps to know exactly what you’re looking for. For example, it took Mel Fischer 16 years of hunting before he finally pinpointed the location of the Nuestra Señora de Atocha, a Spanish vessel that sank in 1622. There, 56km off the coast of Key West, Florida, were most of the remains, including over 40 tonnes of silver and gold, 100,000 Spanish coins and Colombian emeralds. He spent his booty setting up a company that offers diving holidays for tourists — in the knowledge that more of the ship’s treasure was still out there — and in 2011 someone discovered an antique emerald ring worth an estimated $500,000.

Discoveries are occurring regularly, and with improved technology it’s getting easier to find treasure. Far from being salty sea dogs with an old map and a crew of rum-drinking lags, it’s now a billion-dollar industry using the latest hi-tech tools. Odyssey Marine Exploration is a publicly-listed company that specialises in deep-water recovery, and in 2001 it successfully completed a five-year search for HMS Sussex. She went down in 1694 off the coast of Gibraltar while transporting around 10 tonnes of gold to buy-off the Duke of Savoy in the war against France. Odyssey struck a deal with the British government to share the $500million spoils, but — as is often the case with modern treasure hunting — it is now tied up in a legal dispute as the Spanish government is also claiming rights.

Salvage laws are especially complicated, and years of legal wrangling are typical, but the rewards are so great they’re more than worth enduring. Odyssey’s share price increased 60 percent on announcement of the discovery. The same company also recovered 17 tonnes of gold and silver coins worth $400million from the wreck of Merchant Royal, an English ship that sank near the Isles of Scilly in 1641 while returning from Mexico.

Odyssey, however, is not the only company in the business. Sub Sea Research from Maine in the U.S. announced last year that it had found British cargo ship SS Port Nicholson that had been sunk by a German U-boat off the coast of Cape Cod in 1942. She was carrying more than 70 tonnes in platinum bars (valued at the time at $53 million) as well as gold bullion and diamonds. This was a payment to the U.S. from the Soviets for war supplies in WWII. Sub Sea Research finally pinpointed her location using sonar and obtained the salvage rights to the wreck.

The ownership rights, however, are still unclear, though Sub Sea will get something substantial. The HMS Edinburgh was also sunk in 1942 while transporting gold as payment to the allies. England, the U.S. and the Soviet Union had claims to the riches, and in the end the treasure hunters who salvaged her kept 10 percent of the $2million haul.

The good news, if you want in on the action, is that there are more famous treasure-laden ships ready to be salvaged. The 16th century Portuguese ship Flor de la Mar sank in 1511 off the coast of Sumatra. She had 60 tonnes of gold stolen from the Malacca kingdom, in what is now Malaysia. Despite coming to rest in some of the best diving waters on Earth, she is still waiting to be found.

There is also the Spanish galleon San José, one of three ships that the British navy sank in 1708, and was carrying around 300 tonnes of gold and silver coins, as well as chests of emeralds. She went down off Baru Island, near to Colombia, in about 300m of water. The estimated value of her load is more than $1billion.

It seems that pretty much anything that sinks attracts the attention of treasure hunters — legally or not. Maritime author Robert Marx told British newspaper The Telegraph that the Mafia has special teams whose job it is to trawl for sunken treasure. The recently sunken ship The Costa Concordia went down with diamond-studded jewels and other valuables onboard. “As long as there are bodies in there, it’s considered off base to everybody because it’s a grave,” Marx said. “But when all the bodies are out, there will be a mad dash for the valuables.”

For Esquire magazine