The US Virgin Islands have been important trading centers in the Caribbean for more than 300 years. Christopher Columbus explored and named the islands in 1493, and they quickly became a haven for pirates intercepting traffic between Spain and the New World. Rumors of buried treasures remain on Thatch Island and Rendezvous Bay, where pirates gathered to assault rich ships on their way to the busy mercantile port at St. Thomas. With increasing prosperity, merchants forced the pirates out of business, or into more acceptable forms of enterprise. Claimed alternatively by the English, Dutch, French, Spanish and the Knights of Malta, the islands have varying histories. The Danes colonized St. Thomas in 1672 and St. John later in 1718, and bought St. Croix from the French in 1733. Throughout most of the 18th Century, the Virgin Islands thrived on their plantation economy, exporting great quantities of muscavado sugar, rum and cotton. The United States bought the islands in 1917 for $25 million in gold, as part of its military defense strategy. During World War II, the Navy maintained a submarine base in St. Thomas. It was not until well after the war that everyone started to realize the true appeal of the islands. In 1955, there were 5,000 visitors to the US Virgin Islands. Today, that number is more than two million per year.