The Lagoon of Venice is the most important survivor of a system of
estuarine lagoons that in Roman times extended from
Ravenna north to
Trieste. In the fifth and sixth centuries, the Lagoon gave security to Romanised people fleeing invaders (mostly the
Huns and the
Lombards). Later, it provided naturally protected conditions for the growth of the
Venetian Republic and its
maritime empire. It still provides a base for a
seaport, the
Venetian Arsenal, and for
fishing, as well as a limited amount of
hunting and the newer industry of
fish farming. The Lagoon was formed about six to seven thousand years ago, when the
marine transgression following the
Ice Age flooded the upper Adriatic coastal plain. Deposition of river sediments compensated for the sinking coastal plain, and coastwise drift from the mouth of the
Po tended to form sandbars that closed tidal inlets. The present aspect of the Lagoon is the result of human intervention. In the fifteenth and sixteenth centuries, Venetian hydraulic projects designed to prevent the lagoon from turning into a marsh reversed the natural evolution of the Lagoon. Pumping of
aquifers since the nineteenth century has increased
subsidence. Many of the Lagoon's islands had originally been marshy, but a gradual drainage programme rendered them habitable. Many of the smaller islands are entirely artificial, while some areas around the seaport of the
Mestre are also reclaimed islands. The remaining islands—-including those of the coastal strip (
Lido,
Pellestrina and
Treporti)—-are essentially
dunes. Venice Lagoon has been inhabited from the most ancient times, but it was only during and after the
fall of the Western Roman Empire that people coming from the
Venetian mainland settled in numbers large enough to found the city of
Venice. Today, the main cities inside the lagoon are Venice (at the centre of it) and
Chioggia (at the southern inlet);
Lido di Venezia and
Pellestrina are inhabited as well, but they are considered part of Venice. However, most of the inhabitants of Venice, as well as its economic core (its airport and harbor), are on the western border of the lagoon, around the former towns of
Mestre and
Marghera. There are also two towns at the northern end of the lagoon:
Jesolo (a famous sea resort) and
Cavallino-Treporti. ==Ecosystem==