Muri Lagoon stretches across 2.5 square kilometers of Rarotonga's southeastern coast, a shallow turquoise basin separated from the Pacific by a broken reef line that runs parallel to shore for nearly three kilometers. The lagoon floor rarely exceeds three meters in depth, and its sand-bottomed interior holds water so clear that coral heads appear suspended in air from the beach. Four uninhabited motus—Koromiri, Oneroa, Taakoka, and Motutapu—punctuate the reef, their coconut palms marking the boundary where lagoon meets open ocean. Polynesian navigators used these islets as waypoints for centuries, reading current patterns and bird flight to guide outrigger canoes between the Cook Islands archipelago. The lagoon's ecosystem depends on tidal exchange through reef channels that flush nutrients twice daily, sustaining populations of parrotfish, butterflyfish, and sergeant majors that congregate around bommies—isolated coral formations rising from the sandy floor. Porites and Acropora corals dominate the shallow zones, their calcified skeletons providing habitat for sea cucumbers, brittle stars, and juvenile reef fish. Giant clams, some exceeding 40 centimeters in diameter, anchor themselves to coral rubble, their mantles displaying iridescent blues and greens produced by symbiotic zooxanthellae. The water temperature holds steady between 24 and 28 degrees Celsius year-round, eliminating thermal stress that bleaches reefs elsewhere in the Pacific. Visibility routinely exceeds 20 meters on calm mornings, when easterly trades drop and the lagoon surface turns to glass. Muri Beach provides the primary land access point, a public foreshore where coconut palms lean toward the water and fishing boats rest on the sand between trips. The beach runs for 1.2 kilometers along the lagoon's western edge, backed by low-rise accommodations and the coastal road that circles Rarotonga. Local operators have launched snorkel tours from this shore since the 1970s, when the Cook Islands opened to scheduled air service and visitor numbers began their steady climb. The lagoon remains open to all—no entrance fees, no restricted zones—and its shallow depth makes it accessible to swimmers of every skill level. Morning departures typically launch between eight and ten, when light angles penetrate deepest and marine life is most active. Outrigger canoes and motorized craft ferry passengers to the outer reef, where channels cut through the coral barrier and pelagic species sometimes venture into the lagoon on incoming tides. The experience centers on direct engagement with the reef: mask, fins, and snorkel are the only equipment required, and the water's buoyancy does the rest. Guides point out octopuses tucked into crevices, moray eels coiled beneath ledges, and the occasional eagle ray gliding over sand flats, but the lagoon yields its details to anyone willing to look closely and move slowly through the shallows.