They withstand tremendous waves and storms, sending mysterious light signals over the sea at night. They warn of dangerous shallows and guide ships into the harbor. Countless sailors owe their lives to them. Lighthouses shape our coasts worldwide and hold a special fascination for us. "Guardians of the Sea" tells of the efforts and technical achievements that made it possible to construct enormous and stable structures even on the smallest rock outcrops and in the middle of the sea. It describes the most important element of the towers - the light - and its evolution from a weak candlelight to powerful, focused beams. It recounts shipwrecks and heroic sea rescues, and not least, the great responsibility and isolated life of the lighthouse keepers. Historical construction.