A lighthouse is a tower, building, or other type of structure designed to emit light from a system of lamps and lenses, and to serve as a navigational aid for maritime pilots at sea or on inland waterways. Lighthouses mark dangerous coastlines, hazardous shoals, reefs, and safe entries to harbors, and can assist in aerial navigation. Once widely used, the number of operational lighthouses has declined due to the expense of maintenance and use of electronic navigational systems.