The Venetian Lagoon is a very shallow, crescent-shaped coastal lagoon off the northern part of the Adriatic Sea. It is the largest Italian wetland and a major Mediterranean coastal ecosystem. In addition to Venice, which sits on a small island at the center, the lagoon contains many other islands, most of which were marshy but have now been drained. Its average depth is just 28 inches (70 cm), so most boats cross the lagoon only via dredged navigation channels, and four-fifths of its area consists of salt marshes and mudflats. It takes in both riverine fresh water and seawater, and its tides have a range of up to 3 ft (1 m). During periods of spring tide, Venice is regularly flooded (see Global warming and sea-level rise). While its inhabitants have coped with this problem for centuries, land subsidence and rising sea levels now pose a major threat to the city and its art treasures. Marine life in the lagoon includes many species of fish (from anchovies to eels, mullet, and sea bass) and invertebrates. Sea birds, waterfowl, and waders proliferate on the many uninhabited islands. Efforts are now being made to reduce industrial and agricultural pollution, including attempts to capture pollutants by means of shrubs planted along the edges of the lagoon.