My best has been a glow worm dive in Providenciales Turks & Caicos in May. A few days after each full moon (and about 50 minutes after sunset) glow worms rise up from the sand to mate. I was at the back of our group and taking pictures. After taking some pics of a huge crab, I looked up towards the group, before I grabbed my dive light, and the darkness had turned into a universe of stars. The ocean was full of tiny yellow/green lights - absolutely beautiful!