Venezia stretches across 118 small islands in the marshy Venetian Lagoon along the Adriatic Sea in northeast Italy. The saltwater lagoon stretches along the shoreline between the mouths of the Po (south) and the Piave (north) Rivers. The buildings of Venezia are constructed on closely spaced wood piles, which were imported from the mainland. Under water, in the absence of oxygen, wood does not decay. It is petrified as a result of the constant flow of mineral-rich water around and through it, so that it becomes a stone-like structure. The piles penetrate a softer layer of sand and mud until they reach the much harder layer of compressed clay. Most of these piles are still intact after centuries of submersion. The foundations rest on the piles, and buildings of brick or stone sit above these footings. The buildings are often threatened by flood tides pushing in from the Adriatic between autumn and early spring.