Kew Gardens

This is a botanic garden in SW London that houses the "largest and most diverse botanical and mycological collections in the world". Founded in 1840, from the exotic garden at Kew Park, its living collections include some of the 27,000 taxa curated by Royal Botanic Gardens, Kew, while the herbarium, one of the largest in the world, has over 8.5 million preserved plant and fungal specimens. The library has more than 750,000 volumes, and the illustrations collection has more than 175,000 prints and drawings of plants. It's one of London's top tourist attractions and is a World Heritage Site, inscribed in 2003.