Maritime archaeology can be divided in a three-tier hierarchy, of which the first tier consists of the archaeology of shipwrecks and wreck sites. In this discipline the wrecking process itself is studied: how does a ship break up, how does a ship float to the bottom, and how do the remains of both ship and cargo decay over time?
The second tier studies the ship as a machine, both in itself and in a military or economic system.
The third tier consists of the archaeology of maritime cultures, in which nautical technology, naval warfare, trade and shipboard societies are studied.
Maritime archaeology has two important advantages over land archaeology. First the remains of ships and cargoes, even organic materials, are sometimes better preserved under water or in bottom sediments. The second advantage lies in the fact that until recently, shipwrecks were usually beyond the reach of human intervention or salvage, thereby creating perfect time capsules.
In the Mediterranean area, maritime archaeology mainly deals with the innumerable retrievals of ancient ages, especially regarding the Roman fleets. The many discoveries in the sea and in some lakes (notably in Nemi, Italy, where Caligula's ships were found) were really helpful in explaining some passages of the history of Romans, Phoenicians and Etruscans, and allowed to track respective presences in the related areas.
Italy is indeed one of the most important areas for these studies, with particular reference to Roman and Etruscan naval activities. Also because of the extremely high rate of expected wrecks (Romans calculated that at least 30% of cargo would have been lost by storms or pirate assaults), the traffic was proportionally (or perhaps more) increased, and many goods were found (ordinarily contained in amphoras or in the larger dolia) that let us understand what the commerce was about. Sometimes, as in the case of the two "bronzi" found in Riace (Calabria), real artworks were brought to the surface. In other cases, like the very recent retrievals in Sarno river (near Pompeii), other details enlarge the knowledge of some interesting elements: this retrieval allows us to suppose in fact that on the Tyrrhenian shore too there were little towns with palafittes, like in ancient Venice. In the same area, the submerged town of Puteoli (Pozzuoli, close to Naples) contains the "portus Julius" created by Marcus Vipsanius Agrippa in 37 BC, later sunk due to bradyseism.
The Antikythera mechanism, which appears to be an ancient clockwork astronomical computer, was discovered in a shipwreck off the Greek island of Antikythera.
But other areas too have no less interest, like the waters around Israel, where Herod's port at Caesarea was found. Other finds are consistent with some passages of the Bible (like the so-called Jesus boat, which appears to be similar to those in use during the first century CE).