Boaters found its cargo of hundreds of amphoras in 2000 when their anchor got tangled with one of the two-handled jars.
After years of arranging financing and crews, exploration of the site off the coast of Alicante in southeast Spain began in July, said Carles de Juan, a co-director of the project, who works for the Valencia regional government.
The ship is estimated to have been 100 feet long with capacity for around 400 tons of cargo, making it twice the size of most other Roman shipwrecks found in the Mediterranean, de Juan said in an interview with The Associated Press.
The freight was an estimated 1,500 well-preserved clay amphoras, or two-handled jars, used in this case to hold fish sauce — a prized condiment for wealthy Romans, he said.
For centuries the meter-tall amphoras lay undisturbed but for an 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 one 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," de Juan said. "It is an exceptional find."
The last time a ship of this size and quality emerged was in 1985 off Corsica, he said.
Javier Nieto, director of the Center for Underwater Archaeology of Catalonia and not related to this project, also called it immensely important because of the fine condition of the cargo. No other Roman shipwreck is currently under study in the Mediterranean, he added.
"For archaeologists, 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 must have been ferocious because it is odd for such a vessel to have been so close to shore.
"The crew did not care about the cargo or money or anything. They headed for land to save their lives," de Juan said.
De Carles and the other co-director of the project, Franca Cibercchini of the University of Pisa in Italy, presented their first academic report on the site at a marine archaeology 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. This forced the Valencia government to build a thick metal grating to cover the remains and protect the jars.
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.