If there is a lifelist for snorkeling, Palau's Jellyfish Lake has to be on it. Tourists can swim through the upper levels of the water as millions of golden jellyfish migrate across the marine lake following the sun. They swim east in the morning, west in the afternoon, and then at night they dive down to pick up nutrients for the next day.
At some point, likely 12,000 to 15,000 years ago, sea levels rose enough to bring the larvae of jellyfish into the limestone pocketed lakes of Palau. The limestone allows ocean water to seep through, but prevents larger marine mammals from entering the lake. There are 70 marine lakes through the archipelago, but not all of them are stocked with jellyfish.
The often told story about Jellyfish Lake—Ongeim'l Tketau to the locals—is that the animals in here have lost their stinging capabilities, but that's not entirely true. If you want to feel their poison, simply let a jellyfish drift toward your lips, which are sensitive enough to react to their sting. The jellyfish otherwise use their stingers to feed on zooplankton.
For more on Jellyfish Lake, read "Darwin's Jellyfishes," from The National Wildlife Federation.