Jellyfish Lake. Palau
Most people have never even heard of the Republic of Palau in Micronesia, let alone the Jellyfish Lake, home to millions of jellyfish that slowly migrate across the water each day, following the path of the sun. Although obtaining a pass to visit the island is not difficult and really just a matter of money ($100), tourists are only allowed to snorkel among the golden jellies and moon jellies, jellyfish that are normally harmless despite their appearance. Scuba diving is forbidden as the lower level contains dangerous amounts of hydrogen sulfide, which can be deadly.
