Rising dramatically out of the Pacific Ocean 535km southwest of the Costa Rican mainland, PARQUE NACIONAL ISLA DEL COCO (t 2258-8570 or t 2250-7295) is revered among divers, biologists and treasure-hunters. Gigantic waterfalls plunge off jungle-strewn cliffs straight into an underwater world that has made this national park a veritable “Costa Rican Galapagos”. It’s the only island in this part of the Pacific that receives enough rain to support the growth of rainforest and is home to 150 endemic species that are found nowhere else in the world, including the Cocos flycatcher and the Cocos gecko. In addition, more than 250 species of fish – including one of the world’s largest concentrations of hammerhead and white-tipped reef sharks – patrol the surrounding waters. The rugged, mist-shrouded volcanic island itself appeared as “Dinosaur Island” in Steven Spielberg’s blockbuster Jurassic Park: in the opening frames of the film, a helicopter swoops over azure seas to a remote, emerald-green isle – that’s Coco.

Nearly 25 square kilometres in size, Isla del Coco is one of the world’s largest uninhabited islands, yet few would be able to locate it on a map. Perhaps that’s why pirates found it such a perfect hideout during the seventeenth and eighteenth centuries. Legend has it the golden spoils from fruitful church-looting expeditions to Lima were buried here; known as the “Lima Booty”, the stories sparked a frenzy of treasure-seeking missions. More than 500 tried their luck (and failed) before Isla del Coco was declared a national park in 1978, ending all gold-digging expeditions. Though evidence suggests that the island was known by pre-Columbian seagoing peoples from Ecuador and Colombia, in the modern age it was “discovered” by the navigator and sea captain Joan Cabezas in 1526. Attempts were made to establish a colony here in the early twentieth century, and nowadays wild descendants of the would-be settlers’ pigs and coffee plants have upset the island’s ecosystem.

Today, however, conservation is the order of the day on this UNESCO World Heritage Site, although illegal fishing, shark-finning in particular, within the 15km restriction zone is rife, and park rangers and marine organizations lack the resources to bring it under control. Despite this, Isla del Coco remains an increasingly coveted destination for experienced scuba divers. More than a thousand a year brave the gut-wrenching 32-hour boat journey from Puntarenas to spend a week or so moored in the island’s sheltered harbour on live-aboard boats. The subterranean treasures range from underwater caves and technicolour coral reefs to schools of manta rays and, off the northeastern side of the island, the occasional whale shark. The real danger here, however, is not sharks but strong currents, and divers often wear gloves to grip onto rocks to stop themselves from drifting away. Water temperatures are a balmy 22–26°C and the best time of year for seeing sharks is the rainy season (May–Nov). Two sheltered bays provide access to the island itself, and during the day visitors can venture onshore to hike the steaming tropical forests.

Two diving companies make the trip year-round from Puntarenas: packages with Agressor (t 2289-2261 or US t 1-800/438-2628, w http://www.aggressor.com) start at $3600 for eight days, while Undersea Hunter (t 2228-6613 or t 1-800/203-2120, w http://www.underseahunter.com) runs similar expeditions, with ten-day trips from $4750.

For more information on the island, contact the Fundación Amigos de La Isla del Coco (t 2256-7476, w http://www.cocosisland.org), which was founded in 1994 to help preserve the unique terrestrial and marine biodiversity of Coco.