16. Surtsey, Iceland

As one of the world's youngest islands, Surtsey, Iceland, appeared after volcanic eruptions from 1963 to 1967. People are forbidden to visit the island except for a few researchers because they want to figure out how ecosystems form without human influence. 

Scientists have observed fungi, molds, birds, and invertebrates. They can't bring anything from the mainland with them because it could disrupt their research. Although it is a UNESCO World Heritage site, you won't be able to check this one off your bucket list.