The island is also the site of one of the world's largest salt mines, currently operated by the Cargill corporation. Salt extraction has occurred on the island for at least several hundred years, the first benefactors of its salt deposit being American Indians who boiled briny spring water to extract the mineral. In 1862, during the Civil War, the Avery family discovered extremely pure solid rock salt just below the island's surface. Because of a Union blockade, the South had no reliable source for this valuable commodity. As a result, the Averys mined the deposit to supply much of the lower South with salt.
Significance: The Steam Hoist is the oldest still operating in the United States and has a history of ninety years of continuous use on the same site. The Breaker Building is one of the few surviving examples of this type of timber-framed industrial structure. The Salt Works is part of the site of the oldest salt mine in the United States.