Bajo Alcyone

Bajo Alcyone is the most celebrated dive site on Cocos Island — a submerged seamount discovered and named by Jacques Cousteau's Calypso expedition in 1987, rising from the abyss to within 25 meters of the surface 1.5 kilometers offshore from the main island. The seamount is the single most reliable site in the world for scalloped hammerhead shark encounters, with schools of 100-300 hammerheads routinely observed during the peak June-November season and individual sharks visible on virtually every dive year-round. The dive is performed along the seamount's top at 25-30 meters, with divers positioning themselves in the current shadow on the upcurrent side of the seamount to observe the resident hammerhead school patrolling the cleaning stations. The hammerheads use the seamount as a daily cleaning stop — reef fish including king angelfish and barberfish serve as cleaners, and sharks rotate through at close range. Galápagos sharks, silky sharks, and the occasional whale shark or tiger shark also appear, along with giant manta rays during plankton-rich tides. Beyond the sharks, Bajo Alcyone is extraordinary. Marble rays form schools of 30+ individuals resting on the sandy patches between rocks, tuna schools in their hundreds cycle through the seamount, and the resident Galápagos shark population (the eastern Pacific's large-bodied reef shark) patrols the deeper flanks. Whale shark encounters, though unpredictable, are possible from May to December. Bajo Alcyone is technical diving adjacent — strong currents, depths to 30-40 meters, cold thermoclines (water can drop to 19°C below 25 meters), and offshore position make Advanced Open Water with Nitrox and drift experience essential. Access is exclusively by liveaboard from Puntarenas, Costa Rica (36-hour crossing each way, 7-10 day itineraries). Visibility highly variable (15-30 meters, sometimes less in plankton blooms that attract hammerheads). Water temperatures 21-27°C.