Bajo Alcyone
Depth is the range from the top of the site to its usual maximum; "recommended cert" is the mainstream-agency level operators usually ask for, not a legal rule. Currents and visibility change daily — treat everything here as planning ranges.
Located in the late 1980s by Jacques Cousteau's team and named for their turbosail ship Alcyone, then made famous by decades of shark research, Bajo Alcyone is the purest expression of what Cocos does. The seamount's summit levels off around 27 m; divers pull themselves down the mooring line through the blue, wedge in among barnacle-crusted rocks, and wait. Barberfish rise to meet incoming scalloped hammerheads, and on the good days the school builds overhead until it dims the surface light. Marbled rays sail past like flying carpets, whitetips thread the rocks, and the drifting deco stop in open water regularly produces silkies and, in season, a whale shark.
The moment you come for: The archetypal hammerhead cleaning station, marbled rays, blue-water safety stops with silkies
Topography: Flattened volcanic summit at 25-30 m dropping into deep water
Current: Descend the line, get behind rock, stay low; the current is what delivers the sharks.
Know before you splash
Hazards: Depth plus current plus blue-water ascent - a dive to plan conservatively, 36 hours from help.
Conservation: Cocos Island National Park; no gloves on the reef, no contact, ranger-monitored moorings.
Nitrox: Worth it here — profiles sit in the range where enriched air meaningfully extends no-stop time (with the training to use it).
Who you'll meet
Marbled Ray
Scalloped Hammerhead
Silky Shark
Whale Shark
Whitetip Reef Shark
For photographers
Wide angle: ★★★★★ · Macro: ★☆☆☆☆ — editors' 1–5 rating of what this site gives each style of shooting