Photo by Tom Burns

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Along with its legendary propensity for forming immense schools, the scalloped hammerhead (Sphyrna lewini) has a reputation among scuba divers as the shyest shark. So while anticipating my adventure to Cocos Island, my hopes for good shark diving karma centered on an overhead pass by a distant hammerhead school, perhaps a silhouetted memory of a lifetime.

Like many, I had heard stories of elusive, often difficult encounters with scalloped hammerheads on an open-circuit, resulting in the reported need for complicated rebreather units. My expectations had become dismal for an up-close encounter with this characteristic predator. As it turned out, however, when schools of hammerheads came to be cleaned, you got a close buzz of a lifetime - even if you were blowing bubbles.

After diving with countless sharks the world over, I realized that many species were frequently apprehensive of close range encounters. From the untrustworthy look of a great white, to the watchful eye of a massive tiger shark, even the notorious species were often exceedingly cautious. Of course, any seasoned shark diver has had some all-too-close encounters; several kinds of shark will become the large apex predator of lore when the moment is right. But never did I think scalloped hammerheads would have the confidence to move in close with scuba divers. I was wrong, when the hammerheads come in, they come in. Cocos Island is one of several unique locations worldwide where scalloped hammerheads routinely school in large numbers. As I later found out, it is also a place where they come in close proximity to divers.

Cocos Island lies 300 miles southwest of its host country of Costa Rica, a 36 hour boat ride into the privileged loneliness of the eastern tropical Pacific. This UNESCO World Heritage Site has a storied history of whalers, buccaneers, and treasure, but its real wealth lies in the quantity and biodiversity of its marine life. From sharks, rays, and bony fish, to invertebrates and marine mammals, Cocos deserves its reputation as one of the world's most prolific marine sanctuaries.

Along with a myriad of other shark species - including whitetip reef, silky, silvertip, and whale sharks - numerous scalloped hammerhead sharks congregate off Cocos's deep reefs. The scalloped name originates from prominent indentations along the wide leading edge of the head, a region known as the cephalofoil. This head appears "scalloped" and on close inspection differs from the seven other members of the hammerhead shark family. Compared to conventional shark anatomy, hammerheads likely enjoy a unique biological advantage from their distinctive look. The much wider distribution of senses - such as electrical detection, increased lift and maneuverability - could support a strong Darwinian argument that the hammerhead is the best anatomically designed of all the shark species.

During the night, the sharks are presumed to head into deeper waters, extending 40 to 50 miles out, to hunt their prey of fast moving squid and fish. In the morning, they often return in large schools, following the current from the deep blue. Several theories suggest hammerheads may school, in part, for mating purposes, although the complete significance of schooling behavior is still unknown.

Using geological formations as likely navigation aids, the sharks return to Cocos via seamounts and rock outcroppings. Often their most urgent business is a visit to several underwater locations that serve as busy cleaning stations. Diligent barberfish and king angelfish make the rounds as opportunistic cleaners; in return for the prompt ridding of irritating parasites on the sharks, the fish receive a ready and safe meal. As the two parties go about their symbiotic business, they clearly communicate with a common language of body posturing.

Forget the rebreathers. When hammerheads return from their deep hunting escapades to be cleaned, if their mood is right, you may be in for a close-up treat. The best encounters start by divers spreading themselves around up current station locations at a depth of 80 to 120 feet. As the hammerheads materialize from the blue, some conventional scuba divers extend their breathing pause as the sharks approach directly overhead. When the sharks are coming in to be cleaned, however, individual sharks may cruise right into your new neighborhood, regardless of the bubble trail.

For the lucky diver, the experience becomes an almost limitless parade of curious fly-bys, with large black eyes coolly checking you out. When conditions peak at sites like Alcyone and Dos Amigos, the sharks seem totally untroubled by flash photography, and even obvious diver presence. The up-close experience provides an intimate appreciation of the hammerheads, from the cephalofoil in action, to clear views of mating scars and flawless rows of bright white teeth. It could almost make you forget the other outstanding diving opportunities around Cocos, which also features majestic silvertips, crowds of eagle rays, pushy silky sharks, friendly morays, and pods of dolphins.

This close interaction could make the hammerhead sharks of Cocos Island appear almost indifferent and unaffected by humans. The unfortunate truth is, however, that the sharks surrounding Cocos might be the most vulnerable. At this moment, just beyond the patrolled 12 mile border to this World Heritage Site, fleets of long line fishing vessels continue to rob sharks of their fins, with inhumane treatment and waste. Recent studies have shown dramatic population decreases in many shark species, with the hammerheads among the most depleted. The global community needs to preserve the dwindling shark resources before they reach point of no return, so that the schools of hammerheads can continue to follow their ancient routes, and future divers can get a close range encounter with one of nature's most spectacular shows.

Editor's Note: This article was first published in Shark Diver magazine.