A handout picture released by the Tunisian National Heritage Institute and the University of Sassari on August 31, 2017 shows archaeologists diving off the coast of Nabeul in northeastern Tunisia at the site of the ancient Roman city of Neapolis

Vast underwater Roman ruins have been discovered off northeast Tunisia, apparently confirming a theory that the city of Neapolis was partly submerged by a tsunami in the 4th century AD.

"It's a major discovery," Mounir Fantar, the head of a Tunisian-Italian archaeological mission which made the find off the coast of Nabeul, told AFP.

He said an underwater expedition had found streets, monuments and around 100 tanks used to produce garum, a fermented fish-based condiment that was a favourite of ancient Rome.

"This discovery has allowed us to establish with certainty that Neapolis was a major centre for the manufacture of garum and salt fish, probably the largest centre in the Roman world," said Fantar.

"Probably the notables of Neapolis owed their fortune to garum."

Fantar's team started work in 2010 in search of the port of Neapolis but only made the breakthrough find of the ruins stretching out over 20 hectares (almost 50 acres) this summer thanks to favourable weather conditions.

The discovery also proved that Neapolis had been partly submerged by a tsunami on July 21 in 365 AD that badly damaged Alexandria in Egypt and the Greek island of Crete, as recorded by historian Ammien Marcellin.

© 2017 AFP