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.

Ancient lighthouses
Graphic reconstruction of the Pharos according to a 2006 study
The Tower of Hercules lighthouse

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Before the development of clearly defined ports, mariners were guided by fires built on hilltops. Since raising the fire would improve the visibility, placing the fire on a platform became a practice that led to the development of the lighthouse. In antiquity, the lighthouse functioned more as an entrance marker to ports than as a warning signal for reefs and promontories, unlike many modern lighthouses. The most famous lighthouse structure from antiquity was the Pharos of Alexandria, Egypt, although it collapsed during an earthquake centuries later.

The intact Tower of Hercules at A Coruña, Spain gives insight into ancient lighthouse construction; other evidence about lighthouses exists in depictions on coins and mosaics, of which many represent the lighthouse at Ostia. Coins from Alexandria, Ostia, and Laodicea in Syria also exist.