15. Niihau Island, Hawaii, USA

Most people associate Hawaii as a top vacation destination, but there is one island where no visitors are allowed. Niihau Island is also known as Forbidden Island because Elizabeth Sinclair, a Scottish farmer and plantation owner, purchased it back in 1864. Since then, it's privately owned by her family. 

In 1952, Hawaii had a polio outbreak, and to stop the spread, no one was allowed to leave or enter the island. Luckily, no one got sick on Niihau. To this day, people need special permission to visit the island, but it isn't easy even for the rich and famous.