The radio crackles as you slice through the waves toward the outer reef at Ningaloo: the spotter plane has made a sighting. You check your mask, and gaze down to see a speckled torpedo glide past, accompanied by a retinue of smaller cobia and remora: whale shark, the world's biggest fish, growing to over 12m long. Seconds later you're in the water, trying not to gasp through your snorkel as the bus-sized behemoth extends its mouth in a broad gape, hovering up plankton as its tail sweeps languidly. Mesmerise, you gaze at the constellation of spots on its flanks, unique as a human fingerprint.
The whale shark is one of the blue planet's most magnificent marine creatures - and the most mysterious: although scientists study their epic migrations covering thousands of miles across the oceans, it's still unknown where they produce their young - a phenomenon that's never been seen. For nearly half the year, large numbers arrive in Western Australia to feed along the outer edge of Ningaloo, one of the world's longest fringing reefs. Small numbers of operators are licensed to lead whale-shark swims, during which you'll also snorkel above kaleidoscopic coral gardens and possibly spot manta rays, humpbacks, dugongs, spinner dolphins and green, loggerhead and hawksbill turtles.
Top tips: Humpback whales migrate past Ningaloo from around June to October. Manta rays are present year-round, but numbers near Exmouth rise between May and November.
Also try: Donsol Bay, Philippines; Mafia Island, Tanzania; Maldives; Nosy Be, Madagascar.