Suddenly an onlooker spots a dark shadow and the shout goes up: "Shark! Everybody get in the water!" At Donsol, the whale-shark capital of the Philippines, the aim is getting close to sharks, not getting away.
Fishermen have known about the butanding that gather in the watery depths off the coast of Bicol for centuries, but the transformation of Donsol from sleeping fishing village to whale-shark spotting centre is a more recent phenomenon. In 1998, a Filipino dive team spotted startling numbers of whale sharks and alerted the World Wildlife Fund, and it set the wheels in motion for one of the most successful community tourism projects in Asia.
Historically, butanding were viewed with fear by local fishermen, who noted their resemblance to deadlier denizens of the deep. However, the involvement of marine experts led to an official ban on fishing for whale sharks, and a new-found respect for them among Donsol residents. With money coming in from managed shark dives, butanding suddenly had a greater value alive than dead, providing a powerful incentive for conservation.
From the outset, diving at Donsol was promoted with the welfare of the whale sharks in mind. Scuba diving was banned – sharks are spooked by the noise of bubbles from diving equipment – and swimmers were allowed to enter the water only with a mask and snorkel, in small, managed groups. Without tanks and regulators, the encounter is more intimate; you are a visitor in their world.
While Donsol is firmly on the international diving map, this is not your average diving hub. It remains a sleepy fishing village – aside from the visitor centre, a scattering of Filipino-style resorts and the occasional jeepney, there is little to disturb the peace. In the evening, nightlife takes the form of beers on the balcony and firefly-spotting cruises on the local creek.
The adventure unfolds
There's a tangible excitement in the air as the pump boat chugs out from Donsol. For one thing, there's the knowledge that you have to be ready to leap into the water at a moment's notice when the attending butanding interaction officer (BIO) spots a cruising whale shark. Then there's the thrill that comes from knowing you'll be sharing the water with the world's biggest fish.
As the boat skips across the waves, everyone is on tenterhooks, scanning the surface of the water for telltale eddies or fins … then the call goes up: "Jump!" You grab your mask and leap into the water. First there's the jolt that comes from sudden immersion, then your heart skips a second beat as a fish as long as a bus glides beneath you. You maintain a respectful distance, but the shark seems unperturbed by the strange flapping creatures from the surface.
Whale sharks may be filter feeders, but they're unmistakably sharks. Fears evaporate as the graceful ballet unfolds, however. The opening and closing of the giant mouth. The rippling of the gills. The steady sweep of the gigantic tail. Then the shark turns, drops out of view, and melts back into the deep, dense blue.
The focus at Donsol is not on spotting whale sharks, but on "interactions". On a good day, swimmers can encounter a dozen. To put that into context, the great marine explorer Jacques Cousteau saw just two in his lifetime. It's a wild encounter, and there's an element of chance, but in peak season, when sharks gather to feed and breed, sightings are almost guaranteed.
Making it happen
Jeepneys and air-con mini vans run to Donsol from Legaspi, which is served by regular flights and buses from Manila. Peak season for whale sharks is February to May when plankton provide ample food. No dive certification is needed; you just must be able to swim. There are resorts (Giddy's Place is the pick) and centres in Donsol for dives elsewhere along the coast.
* Feeling the spray on your face as the pump boat skims across the waves.
* Glimpsing your first butanding gliding through the silent depths.
* Getting up close and with manta rays off Ticao Island.
* Scouting for fireflies on the Ogod River.
* Admiring the symmetry of Mount Mayon, the world's most perfect volcano, in nearby Legaspi.
Location: Donsol, Sorsogon, Bicol, the Philippines.
Ideal time commitment: Three days.
Best time of year: February to May.
Top tip: Bring your own mask and snorkel for a watertight fit.
This is an extract from 'Great Adventures', from Lonely Planet (£29.99). Readers can buy a copy for £25, including UK P&P, by going to shop.lonely planet.com and using the code INDEPENDENT