Lightships are floating lighthouses that are anchored in areas where it was too deep, expensive, or impractical to construct a lighthouse. Lightships displayed a light at the top of a mast and, in areas of fog, also sounded a fog signal and radio beacon. The fog signals used over the years consisted of bells, whistles, trumpets, sirens, and horns. Early fog horns were powered by steam and later by air compressors. The HURON Lightship sounded her fog horn signal in 3 second blasts every 30 seconds and was known locally as “Old B.O.” because of the familiar sound her horn made.