The wreck site is said to be unusual, because it's in shallow water near the coast.
Such ships usually sank far out to sea where they are almost impossible to locate, de Juan said.
(See a National Geographic magazine feature about searching for Roman shipwrecks.)
The find is also important because of the excellent condition of its cargo, according to Xavier Nieto Prieto, director of Spain's Submarine Archaeology Center of Catalonia.
"For archaeologists, a sunken ship is a historic document that tells us about ancient history and how its economy worked," he told the Associated Press.
The wreck, which lies in 80 feet (25 meters) of water, attracted pirate scuba divers after news of its discovery first spread, forcing authorities to build a metal cage around it.
Around 70 amphoras were reportedly taken.
Such thefts are often a problem, says Roman amphora expert Elizabeth Lyding Will, of the University of Massachusetts at Amherst.
Amphoras are worth about a thousand U.S. dollars each, Will says.
"People put them on top of walls and have them in their houses as decoration," she added.
What Is Garum?
Will says the clay jars are perhaps the most commonly found cargo in Roman shipwrecks.
"They were enormously useful for trade," she said. "You can tell from the shape whether they were for wine, olive oil, or garum."
She says other sauces were stored in amphoras, including one made from tuna hearts, but that garum was the most common type.
The fish sauce was made in Cosa, Italy, before Spain took over the main export trade, producing a much sought-after recipe using mackerel guts.
Pliny the Elder, the first-century Roman scholar, wrote of this sauce: "Scarcely any other liquid except unguents [healing ointments] has come to be more highly valued, bringing fame even to the nations that make it."
Will said, "The Romans liked the Spanish sauce best," adding that there was a lot of trade between Spain and Italy during the period the shipwreck dates from.
Garum figured in many Roman recipes, Will adds.
"Garum was a highly prized condiment and very nutritious, too, but made out of internal [fish] organs," she said. "I'm told it was extremely smelly, but the Romans just loved it."
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