The US Virgin Islands' main islands are St. Croix, St. John and St. Thomas. The gently rolling hills of St. Croix blend with palm-lined beaches and turquoise waters. Old plantation windmills are scattered across the landscape, and the Danish heritage is evident in the towns of Frederiksted and Christiansted. Snorkelers will find their paradise at the nation's only underwater national park, Buck Island Reef. Other popular attractions include a rum-tasting tour of the Cruzan Rum Distillery. On the island of St. Thomas, tourists can explore the undersea world aboard an Atlantis submarine or browse the duty-free shops of Charlotte Amalie. Two-thirds of St. John, the smallest of the US Virgin Islands, is a US national park. Hiking the many trails and relaxing on secluded beaches are popular activities here.