The Bat Islands (Islas Murciélago), a chain of volcanic islands and islets off Costa Rica's Santa Elena Peninsula inside the Santa Rosa National Park marine sector, are the country's bull shark dive: at the Big Scare (El Gran Susto), adult female bull sharks aggregate seasonally at 18–30 m. An advanced, green-season-only day trip — boats run roughly mid-May to early November, and encounters are likely in peak months but never guaranteed.
Destination info
Conditions, highlights, and the resident marine life.
Conditions
Water and air temperature across the year.
WaterAirDryShoulderWet
Description
Islas Murciélago is Costa Rica's only archipelago — five main islands and a scatter of islets trailing off Cabo Santa Elena in the far northwest — protected within the 43,000-hectare marine sector of Santa Rosa National Park, part of the UNESCO-listed Área de Conservación Guanacaste. Recreational diving is restricted to four designated sites, and the headline act is the Big Scare at San Pedrillo islet, where acoustic-telemetry research by the local NGO Misión Tiburón confirmed a seasonal aggregation of adult female bull sharks: over 80% of detections fell between May and November, peaking in July–August. Trips run as long day-boat outings (about 1.5–2 hours each way from Playas del Coco; closer from Cuajiniquil) and only in the green season — December–April brings the Papagayo winds and coastal upwelling that make the crossing rough and the water cool and green. This is honest advanced diving: free negative descents to 20–30 m with no lines, strong and shifting currents, surge and sharp thermoclines, and variable 5–30 m visibility. Beyond the sharks there are devil ray schools that can number in the hundreds, occasional oceanic mantas and whale sharks, dense schools of jacks and grunts, and an unusual mix of habitats — algal-matted rocky reefs, Pavona coral communities and seasonal Sargassum forests — documented by a 2021 scientific expedition.
Highlights
What makes this dive worth the trip.
The bull shark aggregation is scientifically documented: Misión Tiburón researchers acoustically tagged 10 bull sharks at San Pedrillo islet (2013–2015) — all adult females — and recorded nearly 60,000 detections, over 80% of them between May and November with July and August the most active months. The authors concluded San Pedrillo is a temporary seasonal residence likely tied to reproduction rather than feeding.
Diving inside the Santa Rosa National Park marine sector is allowed only at four designated sites — San Pedrillo (El Gran Susto / Big Scare), Los Arcos, Bajo Negro and Bajo Pochote — with bookings through the ACG's official reservation system; landing on the islands is prohibited, and Isla San José's research station and turtle-nesting habitat remain closed to tourism.
The islands sit inside the 43,000-hectare marine sector of the Área de Conservación Guanacaste — Costa Rica's only archipelago — patrolled from a research-and-surveillance station on Isla San José. The protected waters hold true Pocillopora coral formations, octocorals, four visiting sea-turtle species, dolphins, seasonal humpback whales and pelagics from sailfish and marlin to manta rays.
Marine life
24 species you’re likely to encounter on a dive here.
Dive sites
4 signature sites at this destination.
The Big Scare (El Gran Susto)
Costa Rica's signature bull shark dive, at San Pedrillo islet on the outer end of the archipelago — the same islet where Misión Tiburón's acoustic receiver documented the seasonal aggregation of adult females. Divers make a quick negative descent onto volcanic rock and sand at 18–30 m and hold position while bull sharks cruise past, usually below 20 m near the thermocline. Oceanic mantas, devil rays, big jacks and snapper schools round it out. Encounters are most likely July–August but are never guaranteed.
18–30 madvancedDay boatStrongVisibility 8–25 m
Black Rock (Bajo Negro)
A large volcanic pinnacle rising from about 30 m to near the surface roughly a kilometre offshore, usually dived as the second tank of a Bat Islands day. Divers circle the formation through dense, near-constant schools of fish — jacks, grunts, snappers — with whitetip reef sharks, eagle rays and morays in the rock. Gentler than the Big Scare but still open-Pacific diving.
5–30 mintermediateDay boatModerateVisibility 8–25 m
Los Arcos
One of the four officially authorized dive sites of the Santa Rosa marine sector, set off Cabo Santa Elena at the western limit of the reserve — exposed rock beneath the cliffs of the cape, named for its natural rock arches. Big-animal potential is the same as the islands proper (rays, pelagic schools, passing sharks), with octocoral-dressed rocky reef. Exposure to open-Pacific swell makes it a conditions-dependent call by the operator.
18–30 madvancedDay boatModerateVisibility 8–25 m
Bajo Pochote
A rocky shoal off Punta Pochote on the Santa Elena Peninsula's north coast, near the mouth of Bahía Santa Elena — the gentlest of the four authorized dive sites and the only one also designated for snorkelling (position and depths approximate; published site detail is thin). Expect rocky-reef life over sand: reef fish, rays, morays and turtles rather than the big-shark action of the outer islands.
5–18 mintermediateDay boatLightVisibility 6–20 m
Where to dive & stay
Local dive centers, resorts, and hotels.
Featured operators coming soon
Verified dive centers, resorts, and hotels around Bat Islands (Guanacaste) will list here — pricing, photos, and direct contact.
