In HD, underwater footage of the famous Jellyfish Lake in Palau. From wikipedia: 

Jellyfish Lake is connected to the ocean through fissures and tunnels in the limestone of ancient Miocene reef. However the lake is sufficiently isolated and the conditions are different enough that the diversity of species in the lake is greatly reduced from the nearby lagoon. The golden jellyfish… and possibly other species in the lake have evolved to be substantially different from their close relatives living in the nearby lagoons.

Update: If the YouTube video above is blocked from your country, try this copy on Vimeo.