The park began small and grew in concentric rings of protection. It was established in 1977 to safeguard a single coral reef on one of the Islas del Rosario, originally covering just 178 square kilometers. In 1988, an additional 195 square kilometers were incorporated. Then, in 1996, the park expanded dramatically to its current 1,200 square kilometers, absorbing the Archipelago of San Bernardo to the south. This stepwise growth reflected an expanding understanding of how coral ecosystems connect -- that protecting one reef in isolation accomplishes little when currents, sediment, and temperature affect the entire system. Today the park is one of three national parks in the Colombian Caribbean with coral reefs, alongside Tayrona to the east and Old Providence McBean Lagoon on the distant island of Providencia.

The reefs within the park range from depths of one to thirty meters, and they host 167 documented fish species, 18 of which are classified as endangered. Loggerhead sea turtles, green sea turtles, and the critically endangered hawksbill sea turtle nest and feed in these waters. Calcareous algae encrust the reef framework. Soft corals sway in the current alongside sponges, feather duster worms, and sea lilies. Seagrass beds blanket the sandy flats between reef structures, providing nursery habitat for juvenile fish and grazing ground for turtles. Mangrove forests fringe the islands above, their tangled root systems filtering sediment and sheltering crabs, mollusks, and the larvae of species that will eventually colonize the reef. It is a vertically integrated ecosystem -- mangroves at the surface, seagrass in the shallows, hard coral on the slopes, each layer depending on the others.

The park's greatest threat arrives not from the open ocean but from the land. The Canal del Dique, a waterway connecting the Magdalena River to the Bay of Cartagena, carries freshwater and sediment from Colombia's interior directly into the park's waters. The Magdalena drains nearly a quarter of Colombia's land area, and the sediment load it delivers smothers coral, blocks light, and lowers the salinity that reef organisms require. The effect is measurable: reefs nearest the canal's outflow show significantly higher rates of degradation than those farther away. Add warming sea temperatures -- which trigger coral bleaching events -- and the park's reefs face a two-front assault. The Caribbean and Panama currents that shape the park's waters also carry nutrients and plankton, sustaining productivity but bringing additional sediment and pollutants from upstream development.

In 2022, the Colombian government launched "One Million Corals for Colombia," the largest ocean reef restoration project in the Americas. The effort is centered in part on the Rosario and San Bernardo park, where teams place ceramic discs called "cookies" seeded with micro-fragments of coral. Each cookie carries five fragments from eight different species, and the technique accelerates coral growth to roughly 40 times the natural rate. By mid-2023, the project had produced over 735,000 new coral fragments across 12 reef areas in the Caribbean and Pacific. The goal is to grow one million fragments and restore 200 hectares of reef. It is painstaking, underwater work -- divers placing small ceramic discs one by one on degraded reef surfaces, hoping that human patience can outpace the damage that human activity has caused.

For most visitors, the park is a day trip from Cartagena -- a boat ride through the bay, a snorkeling session over shallow reef, lunch on a wooden platform above the water, then back to the city by dusk. That simplicity belies the complexity of what lies beneath. The park protects ecosystems that took thousands of years to build and that are degrading on a timeline measured in decades. Mangrove clearing for tourism development reduces the natural filtration that keeps reef water clear. Boat anchors damage coral heads that grow millimeters per year. Yet the park also demonstrates that protection works: areas where fishing is restricted show measurably higher fish density and diversity than unprotected zones nearby. The challenge is scaling that protection to match the scale of the threats, and the One Million Corals project represents the most ambitious attempt yet to do exactly that.