Last night I dreamt of hammerhead sharks, again. Pale shapes, monstrous and seducing at the same time. They emerged from the deep of my slumber as if rising from the depths of the Pacific Ocean during our dives at Isla del Coco in Costa Rica. They looked like ghosts, swimming in silence

First just one, then two, then hundreds of them, far and close, big and small, rising from the abyss, silhouetted against the light, beautiful and horrible. Swimming with half-open lipless mouths, small and curved teeth glinting and their eyes, on the sides of their flat head, madly staring at you, while others glided close, superb and imperious.

The big hammerheads showed up during every dive: they were like a blink in the haunted, turbid waters, swimming above shoals of hundreds of jacks and trevallies, hovering above dozens of whitetips and great marbled rays.

Sometimes sharks appear in formation from nowhere. We look at hundreds of them, swimming above us while we grab onto cliffs or rocks, in the whirl of a strong sea current which almost sweeps us away. We look hard in the dark blue void, without seeing anything for a long time, nothing but the hissing of the regulator and the gurgling of bubbles. Suddenly we turn round and there it is, an enormous shark, hovering still in midwater, staring at us from less than two metres away.

Taking a good pictureof them is a strenuous and difficult undertaking, due to both their hypnotic charm and the speed of their appearance and disappearance, not to mention the lack of light due to the depth we are swimming at. In fact Isla del Coco (or Cocos Island according to the US spelling), a thirty square kilometers island of untouched rain forest, dizzying gorges, spectacular waterfalls and steep rock formations jutting out at sea (it is located almost thirty nautical miles from the coast) is the perfect place to see hammerhead and silky sharks in large numbers; with a bit of luck you can see them almost on every dive.

Obviously, one has to be careful: hammerhead sharks Sphyrna lewini, which can get almost four metres long, are very shy and to see them well one needs to hide among the rocks at the bottom, even better if close to a cleaning station (here the cleaning fishes are not only the usual wrasses, but butterflyfish Chaetodon and angelfish Holacanthus passer) and patiently wait. Air intake should also be kept at a minimum to avoid unnecessary noise: the use of rebreathers, if available, is strongly recommended here.

But Isla del Coco offers lots of other different, equally spectacular encounters: big eagle rays, mobulas, gigantic Pacific mantas and whale sharks, whose appearance is always obviously welcome. We also experienced the thrill of a fast snorkel dive with a pod of pilot whales and the terrifying sight of an enormous sailfish attacking an immense ball of sardines: a violent and wild business that has once more convinced me that the only fish to be truly scared of are the huge, fast, aggressive "sword bearers". There are also a lot of huge marbled stingrays Dasyatis melanospilos, whitetip reef sharks Triaenodon obesus (which in these waters are really big, but pale to nothing in comparison to the hammerheads) everywhere and lots of their prey, mostly octopi and lobsters.

Unfortunately there is an almost complete lack of colour: in recent times the coral reefs have been almost totally destroyed by the high water temperatures due to El Nino, and consequently all dives are done in a bare rocky environment, brightened only by thousands of starfish Linckia and Oreaster dotting the bare dark rocks everywhere. The persistence of this meteorological phenomenon in the Pacific Ocean has made hammerheads migrate in a rather unpredictable way. With the rising of water temperature they often move to deeper waters for long periods, far away from the preening eyes of visiting divers.

The presence of strong, sudden sea currents makes night diving quite challenging and the possibility of doing macro is rather remote (besides, it is not very likely that first-timers here would decide to take pictures of pretty nudibranchs while dozens of big beautiful sharks are swimming all around). Moreover water is most of the times very turbid and dark, one of the reasons being its richness in plankton. For all these reasons, Isla del Coco is a destination for expert divers only. To enjoy its beauty one has to be driven, and experienced enought to calmly face the strong currents and the big swells of the open ocean. Nevertheless the rewards for those who can do it are remarkable, and the memories will remain forever.

When our boat turns round towards the mainland, we look back, for the last time, at this mysterious island, which has been the destination of pirates, dreamers, mercenaries and treasure hunters for so long. The swirling clouds crowning the top of the island take all sorts of fantastic shapes. They look like hammerheads.

Isla del Coco is the legendary location that inspired Robert Louis Stevenson's novel "Treasure Island". For a good reason: the island has been, for over a century, the destination of pirates, buccaneers, and mercenaries. there are hidden treasures estimated more than a billion dollars worth all over it!

Coco Island was, for a long time, unmarked on nautical charts. It was far away from the mainland and rich in fruit trees and fresh water, so it was used as a hiding place and "buen retiro" by several sanguinary pirates such as Benito Bonito, William Dampier, Edward Davis, Lionel Wafer, Bennet Graham and the famous Morgan, and they all buried their loot inside the island. Apparently, the place also hides somewhere the immense "treasure of Lima", which was carried on the "Mary Dear" in 1821 and whose value was estimated, at the time, at more than 60 thousands dollars.

Many people have hunted these treasures during the last fifty years. Some of them were eccentric daydreamers like the German August Gissler, others were methodical diggers like the American Jimmy Forbes. All of them tried to decode the mysterious incisions engraved on the rocks by the pirates, others have followed the vague indications of ancient maps handed down from generation to generation. There have been some interesting recoveries such as old coins, jewels, dobloons.but nobody has found the big one yet.

Isla del Coco is the best destination for those divers who have seen almost everything and, having had enough of bright colours, easy diving and relaxation, are now looking for something more stimulating. Going to Isla del Coco for diving is real adventure for spoiled divers!

The journey to reach it is long, tiring and expensive, and the dive sites are not particularly beautiful. But it is a fact that Isla del Coco, thank to the incredible encounters that guarantees, will always present divers with unforgettable emotions. Moreover, the island offers the possibility of seeing animals almost impossible to find elsewhere. Due to its isolated position (it's the biggest oceanic island of the world) it is very likely that you'll encounter in its waters big specimens of pelagic fauna such as marlins, sailfish, dolphins, wahoo, tunas, almaco jacks, dolphinfish, enormous droves of trevallies of several different species, mobulas, mantas, eagle rays and sometimes even whale sharks, pilot whales and fin whales.

However, the main attraction of the island are the big hammerheads Sphyrna lewini, often hundreds at a time; the big shoals of silky sharks Carcharhinus falciformis; the numerous whitetip reef sharks Triaenodon obesus - which often get very big here - and the huge marbled rays Taeniura melanospilos. In the nooks and crannies of the submerged cliffs there are also scores of octopi, lobsters and different species of moray eels. Among reef fishes, big shoals of snappers, surgeonfish, squirrelfish stand out; large hawkfish, big groupers, spotted flounders and lots of bright yellow trumpetfishes are very common too.

Corals and sponges are rare, while numerous and rather dangerous (especially at night time, due to the strong currents) are the big tropical sea urchins belonging to the genus Diadema. All dives take place from the big Zodiacs supplied by your liveaboard of choice. These are also used to reach the mainland for a few land excursions or to go out to open ocean at midday hoping for some big chance encounter (snorkellers only).

Use of dive computers and inflatable balloons is mandatory. Dives - normally three during the day, plus an extra one at night time - take place along vertical walls or in shallow waters near Coco and the islets close to it in completely autonomous buddy pairs. No diving courses are performed on board, so the trips are recommended to expert divers only. Should an accident occur, the nearest decompression room is at least a three days journey away and is not always available.

Belonging to Costa Rica and located in the Pacific Ocean at some 260 miles from the coast, Isla del Coco measures 25 square kilometers and is covered in rain forest, waterfalls and small rivers. Completely wild and uninhabited only two researchers and a few guardians live there), it is a world-famous marine and terrestrial reserve and home for very rare species of plants, birds, reptiles and insects.

Costa Rica is one of the best destinations in the world for wildlife and nature lovers. The protected areas (18 parks, 7 biological and 11 faunal reserves) cover, on the whole, more than 1/4 of the entire country. The parks, which are all very well organised, are a maximum of four hours away by car from the capital San Jose, and the motorway network is really good. The only drawback is that excursions are very expensive.

On the Pacific Coast, in the Guanacaste District, Santa Rosa National Park (49.515 hectares) is an international research centre on dry tropical forests and marine turtles. On the Atlantic Coast, Tortuguero Park (18.946 hectares) is the most important spawning area for the Western Caribbean green marine turtles and it also plays host to several species of monkeys and birds. It is possible to reach its beaches by boat, sailing along a tangled network of canals.

Near San Jose the biological reserve of Carara is a rain forest where one can easily see birds (especially beautiful macaw parrots), monkeys, and sometimes jaguars and reptiles. Monteverde is a 10.000-hectare forest and the richest pluvial reserve of the country with more than 400 species of birds (amongst which the quetzal, considered the most beautiful bird of the Americas, stands out), 490 species of butterflies, hundreds of mammals and 2.500 varieties of plants. zxvxzv