Visitors to Komodo National Park enter the dragon reserve on Rinca Island through a gate that is worryingly reminiscent of Jurassic Park. The atmosphere on the other side is tense – the dragons provoke a feeling of primordial discomfort – but the 10-foot-long stars of the terrestrial show look positively minute compared to the beasts that greet the increasing number of divers making the trip to this wild corner of Indonesia. The real titans of the world's deadliest archipelago aren't behind a fence.

Manta rays, which measure up to 23 feet across, congregate in the nutrient-rich water of the Savu and Flores seas, which lap against Rinca, Komodo, and the hundreds of other Lesser Sunda Islands. Divers hop a series of ferries from Bali or book live-aboard cruises in order to watch flocks of these gargantuan eagle rays make balletic passes through clouds of plankton. The mantas even seem to take a dedicated interest in humans, making repeated passes to check out swimmers, who routinely panic despite the fact that these massive animals, which possess no stinging barb on their whip-like tails, are almost totally harmless.

Similarly benign yet intimidating, whale sharks also frequent these waters, breaking the surface with their giant grasping mouths. The sharks pay little mind to divers, who don't seem to register as large enough to warrant consideration. Gray sharks, the enforcers of this blue idyll, are more aware and decidedly more threatening, especially when they swim in ganglike packs.

The thriving coral, shoals of color-flashing squid, hawksbill sea turtles, and banded sea snakes that densely populate these balmy waters are probably the only animals that owe their good health to the irritable dragons back on shore. The national park here was established in 1980 to protect the world's largest lizards and subsequently expanded to encompass the region's aquatic marvels as well. The UNESCO World Heritage Site now contains some of the world's most pristine reefs and naturally abundant waters: Pelagic sharks and massive tuna cruise past communities of Day-Glo nudibranchs and spindly lionfish.

And the dragons – small as they may be – are worth a visit. The deadly predators, whose spittle is so rancid that infections claim most animals they bite, are hard to miss on Komodo. They stalk across the beach and lie in the shade of the park's unassuming headquarters. Because their interest in humans goes well beyond curiosity, guides carry forked sticks to keep them at bay.

Increasingly, dive outfits are pairing trips to Manta-spotting sights with onshore excursions, meaning that travelers devoted enough to make the trip have the opportunity not only to see the last of the terrible lizards but to take in the ultimate megafauna double bill.

More information: Komodo is not an easy place to get to. Our recommendation would be to spend a few nights at the Komodo Resort, which caters specifically to divers, or book a berth on the Felicia, a live-aboard sailboat that offers a bit of piratical luxury for $390 a night.