The first warning of the giants' arrival is a click from the hydrophone - and another, and another, closer and closer together. Then a cluster of vast, dark shapes, like a squadron of submarines, appears beneath the surface. Lowering yourself silently into the clear Caribbean Sea, you hold your breath as the world's largest and loudest toothed predators approach.
Growing to over 18m long, with its distinctive square head, stumpy dorsal fin and slender jaw crammed with some 52 massive teeth, you'll know a sperm whale when you see one. Or, as is the case around Dominica, several: perhaps 300 individuals feed, breed and calve in these waters.
On whale-watching trips around Scotts Head, Roseau, Layou and Point Round, you've a good chance of spotting these monumental carnivores, which dive to depths of over 1km in search of their favourite food: squid. Also look for pygmy sperm whales, short-finned pilot whales, melon-headed whales, and pods of Fraser's, bottlenose, spinner and spotted dolphins.
Top tips: Permits for swimming encounters are limited. Humpback whales migrate past from January through to March.
Also try: Blue whales in southern Sri Lanka; beluga whales in Hudson Bay, Canada; minke whales in west and north Iceland; southern right whales in Peninsula Valdes, Argentine Patagonia.