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Wetlands are areas such as swamps, bogs, and marshes where water either covers the soil or is present at or near the surface, particularly in the root zone, at least a good portion of the year, including the growing season. In the past, wetlands were generally considered unproductive or undesirable lands—smelly and unhealthful, a breeding ground for mosquitoes and other pests—and many were filled in to create farmland or to develop land for housing and industrial use. More than half of the original wetlands in the continental United States have disappeared in the name of reclamation, disease prevention, and flood control. Scientists now realize that, far from being noxious barrens, wetlands play a key role in the ecosystem. They act as filters, removing pollutants, including metals, from waters. They serve as reservoirs, and they aid flood and erosion control by absorbing excess water. Wetlands are home to a great variety of plant and animal species, some endangered, that have evolved to live in the wetland's unique conditions. The preservation and, where possible, restoration of these vital habitats has become a primary goal of environmentalists around the world.