The Commissioners of Irish Lights (CIL) maintain a network of 80 operational lighthouses around Ireland. A lighthouse is a tower, building, or other type of structure designed to emit light from a system of lamps and lenses and used as a navigational aid for maritime pilots at sea or on inland waterways. Lighthouses mark dangerous coastlines, hazardous shoals, reefs, safe entries to harbors, and can also assist in aerial navigation. Once widely used, the number of operational lighthouses has declined due to the expense of maintenance and replacement by modern electronic navigational systems such as strobes.