Lighthouse

From Wikinfo

A lighthouse is a type of building used to warn boats and other watercraft that they are near land and may risk running aground in shallow water (or hit rocks) if they continue to approach. Lighthouses are typified by a tall tower design, sturdy construction, spiral staircase to the top floor, placement near the shore, and a powerful lamp which uses a parabolic mirror to project an easily identifiable beam towards ships. The beam slowly rotates to make sure that it is visible from all positions and to help call attention. Lighthouses also act as convenient landmarks for navigation.

Lighthouse construction faces (or faced) the unique challenge of finding suitably firm ground near the shore to support the large weight. Some lighthouses have also been affected by eroding shorelines, such as the one at Cape Hatteras in Virginia.