The ship, estimated to be 100 feet long with a capacity for around 400 tons of cargo, is twice the size of most other Roman shipwrecks found in the Mediterranean, de Juan said.
Its cargo of an estimated 1,500 well-preserved clay amphoras was used to hold fish sauce, a prized condiment for wealthy Romans, he said.
For nearly 2,000 years, the 3-foot-tall amphoras lay undisturbed except for the occasional octopus that would pry one open, breaking the ceramic-and-mortar seal in search of food or shelter.
Besides the size of the ship and good condition of its cargo, the site is also important because it is so easily accessible -- in just 80 feet of water about a mile from the coast. Other wrecks are so deep they cannot be examined by scuba divers.
"I am not going to say it was on the beach, but almost," said de Juan, who was among the first divers to examine the shipwreck in 2000.
"We knew it was an important find but had no real idea until now," he said. "It is an exceptional find."
The last time a ship of this size and quality was discovered was in 1985 off Corsica, he said.
Javier Nieto, director of the Center for Underwater archeology of Catalonia and who is not related to this project, also called it immensely important because of the good condition of the cargo.
"For archeologists, a sunken ship is a historic document that tells us about ancient history and how its economy worked," Nieto said from Barcelona. "This ship will contribute a lot."
This ship probably sank in a storm while sailing back to Rome from Cadiz in the south of what is now Spain. The storm is believed to have been ferocious because it is odd for such a vessel to have been so close to shore.
De Juan and the other co-director of the project, Franca Cibercchini of the University of Pisa in Italy, presented their first report on the site at a marine archeology conference last week in the town of Gandia, near Valencia.
When word of the find first spread in 2000, pirate scuba divers raided the site and stole some of the amphoras.
What remains of the wooden structure of the ship itself -- about 60 percent -- is buried under mud in the seabed, de Juan said.
The cargo probably also includes lead, which the Romans used for plumbing, and copper, which they mixed with tin to make bronze for everything from plates to jewelry.
The fish sauce is no longer in the amphoras because the seals were not hermetic and could not withstand 20 centuries under water. But traces of fish bone remain inside and these will help researchers determine how the sauces were made, de Juan said.
