The Islas Marietas are often called the Mexican Galapagos. Since 2008, they have been a UNESCO Biosphere Reserve. The islands are a prime viewing area for humpback whales during the winter breeding season and excellent year around for snorkeling and bird-watching, including several endangered species. The Hidden or Lover's Beach can be reached only by swimming through a short tunnel from the sea and is accessible only at low tide. Boat tours from Puerto Vallarta are popular, so book early.