It's just after sunset in Long Beach, California, and John Dabiri stands on the end of a wooden dock, peering down at the water. In his white sneakers and striped polo shirt, Dabiri might be just another boater checking out a well-known local spectacle: a pulsing mass of hundreds of softball-size moon jellyfish that regularly gather here.

Alan Burns breaks the surface with a huge grin on his face, his baggy black wetsuit hanging off his body like walrus skin. It’s a scorching February afternoon, and we’re floating in the clear blue water of the Indian Ocean. To our left is the Australian resort island of Rottnest. To our right—just beyond Burns’s dazzling white yacht—is several thousand miles of open sea. And beneath us, the kelp forest where we had been diving moments before is swaying to the rhythm of the waves.