4. Mafra National Palace Library, Portugal
Nestled in Portugal's Mafra National Palace — a Baroque masterpiece built in the 18th century by order of King João V — is the incredible Rococo library. The spectacular space holds a collection of more than 35,000 leather-bound volumes ranging from the 14th to 19th centuries. Yet beyond its good looks and great books, the library stands out for another reason; a colony of bats is allowed to live in the library, which in turn results in natural pest control of book-damaging insects.