The coins, which date back to the first half of the fourth century, were found on the sea bed not far from the north-east shore of the Mediterranean island.
According to an initial estimate, made on the basis of the overall weight of the find, the number of large bronze coins - called follis - is between 30,000 and 50,000.
Follis coins were introduced around 294 AD by the Roman emperor Diocletian.
They were initially discovered by a private citizen who, during a dive, noticed some metallic remains at a shallow depth and later notified authorities.
