When the last responders leave the sunken cruise liner Costa Concordia, the wreck's status may shift from grave site to treasure trove. Its passengers and interior decorators reportedly left behind a wealth of cash, jewels, antiques, and thousands of pieces of art. Souvenir hunters, looters, and even the mafia may have plans to dive the wreck for a piece of that fortune.

It's been a busy month for shipwreck headlines and shipwreck hunters. The team that announced the discovery of the Port Nicholson, a World War II–era British merchant ship found 50 miles off the coast of Maine, says it bore 71 tons of platinum ingots worth about $3 billion. Other shipwreck hunters turned up the HMS Victory, which sank in the English Channel in 1744 with a "secret" cargo of gold valued at $1 billion. And, in an episode that shows the high stakes of shipwreck salvaging, Spain is currently recovering the estimated $500 million haul of gold and silver from the Spanish galleon Nuestra Señora de las Mercedes that sank in 1804; an American company found the ship but lost court cases to Spain over the rights to the treasure.

All this undersea treasure hunting got us wondering: Just how much money is out there buried at sea? We put the question to marine archeologists, a historian, and a shipwreck hunter. Their answers ranged from "Who knows?" to "$60 billion"—and each was instructive.

How Many Shipwrecks are There?

An estimate of the value of sunken treasure in the world begins with a guess at the number of sunken ships. James Delgado, director of the Maritime Heritage Program at the National Oceanic and Atmospheric Administration (NOAA), estimates that there are a million shipwrecks underwater now.

"Given everything that's charted and all the rest, I would say that the majority of them remain undiscovered," Delgado says. After all, 70 percent of the planet's surface is water, and humans have only begun to be able to reach the depths. "[Considering] this, 95 percent of the ocean still remains unknown to us. It's the last frontier," Delgado says. "We know more about the surface of the moon than what's at the bottom of the sea."

Maritime historian Amy Mitchell-Cook at the University of West Florida says she doesn't think it's possible to make an estimate. "Even in Pensacola Bay, where I am, I don't think we have an accurate number of shipwrecks," she says. "There were Spanish, French, English, and Americans all in the area, as well as international trade. We know a lot of ships sank, but we don't have a complete set of records."

How Much Treasure is Down There?

The short answer, Sean Fisher says, is $60 billion. Fisher is a shipwreck hunter at Mel Fisher's Treasures in Key West (Sean is Mel's grandson), and he gives this figure based on his company's historical research. Fisher specializes in finding Spanish vessels that sank while toting gold, silver, and other loot from the Americas to Europe. And the Spaniards, Fisher says, kept records in triplicate, allowing modern-day shipwreck hunters to see the preserved ship manifests. Those manifests reveal an incredible amount of wealth crossing the ocean, and occasionally sinking into it.

"For about 300 years, the Spaniards came over here and stole all of the wealth of the Americas," Fisher says. "They would lose about 10 percent of that as the cost of doing business. Several wrecks out there by themselves are worth several billion dollars."

Are Shipwrecks Worth the Effort?

Don't be fooled by the flashy dollars figures, Mitchell-Cook argues—we can't know how much wealth is at the bottom of the sea, but it's not enough to make it worth your time.

There's no documentation for many shipwrecks and disappearances, Mitchell-Cook says. And there's no way to know what happened to the wreck. Some ships drop intact in deep water, others run aground, or break up in a storm and scatter their remains for miles. "Also, currents, bottom conditions, temperature of water, and other factors can work to preserve or destroy a vessel," she says. Don't forget fishing, construction, and salvage, which can also disrupt a wreck site.

Delgado, who has excavated wrecks for archaeology institutions throughout his career, says the litany of unknowns is part of the explanation for why wreck hunting is so expensive. "The street value of shipwrecks is minimal. Generally it's a losing proposition, except in a few cases, because it costs more to find them, work them, and deal with what you find. An average mission in deep water can run into the millions, and that's just to find it. For every dollar you spend looking and finding, you'll spend about $10 excavating and treating what you've found [to offset] chemical changes that happen underwater."

And then there's the question of whether you can keep what you find. Governments, insurance companies, and anyone with a chance at a legal claim will set their sights on recovered treasure. Fisher says court cases figure into the projected costs of doing business as a wreck hunter.

In the latest example, one of the biggest names in the shipwreck recovery industry, Odyssey Marine Exploration, lost rights to the gold and silver it had salvaged from the galleon Nuestra Señora de las Mercedes. Odyssey found the wreck off the coast of Portugal in 2007, and the Spanish government sued over the rights to any treasure hauled up. Odyssey lost every court case. This week Spain is taking control of the more than 500,000 coins, which Odyssey had been storing in Florida since it salvaged the treasure.

The Tech to Dive Deeper

Our planet's vast oceans remain remarkably unexplored. But new gear including GPS, refined dive gear, improved sonar, magnetometers, autonomous vehicles, and renavigation software have made it easier to find wrecks and dive to the deep ones.

Most of the still-undiscovered wrecks are deep, Delgado says. To get to them, divers need rebreathers and gas mixtures that extend their range and decrease decompression times. When they're too deep or dangerous for divers, autonomous underwater vehicles outfitted with cameras and excavation tools do the exploring and heavy lifting.

But first you have to know where to look, and the hunt begins with historical research. It's like detective work, Mitchell-Cook says. A maritime historian like her would plug that information into a renavigation program that uses local wind speeds, currents, the sightings of the fleets at the time the ship was lost, and whatever other information is available to narrow a search area where the wreck will most likely be. The better the data, the smaller the area.

Ships search the probable locations with magnetometers and side-scan sonar, either towing a unit or dropping an AUV. Sonar has improved over the years, Delgado says. "Instead of a blurry, pixelated image, we have sonar that's crisp and almost photographic in quality," he says. Images of unnatural shapes such as straight edges or perfect circles could merit closer inspection with remote-operated vehicles and cameras.

A Well-Excavated Shipwreck: Priceless

"All material goods hold value to archaeologists and historians, whether it is a small piece of rope, a broken bottle, or even a nail," Mitchell-Cook says.

The historians we contacted are unanimous in their condemnation of treasure hunters who destroy the historical record. One museum director even declined to talk about the value of undersea wrecks for fear of encouraging looters.

Fisher, the shipwreck hunter, concedes that "a few bad apples" have given his profession a bad name. But his company is diligent in its documentation of the wrecks, he argues. In fact, his paycheck depends on it. "You know, a silver coin is worth the cost of silver if you don't know the history of it," he says.

Case in point: The HMS Victory. Reporting on the discovery earlier this month, The New York Times said that the gold on board could be worth $160 million. But, if it were auctioned based on its historical value, it could fetch perhaps $1 billion.

"The value of shipwrecks is actually priceless in the true sense of the word," Delgado says, "because they tell us our story about our forgotten past. They're not a commodity to be bought and sold; they're something that gives voice to what was forever silenced."

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