When you’re planning a trip, a UNESCO World Heritage designation guarantees a certain wow-factor. Think: the Acropolis in Athens and Angkor Wat in Siem Reap. But for every big-name site, there’s a lesser-known UNESCO world treasure you’ve probably never heard of. Some require a bit more work to get to — their lack of access keeping them out of the spotlight. But with that extra effort comes fewer crowds, and the rare opportunity to feel like you've got the entire place to yourself. From ancient stone cities to remote paradises only accessible by seaplane, here are 21 of the most underrated UNESCO sites in the world.