Reserva Natural Las Cabezas de San Juan (Carr. 987, km 6, 787/722-5882, guided tours Wed.–Sun. 9:30 a.m., 10 a.m., 10:30 a.m., 2 p.m., $7 adults, $2 children 11 and younger) is a unique and treasured piece of island property that has been protected from encroaching development.
This 316-acre piece of land contains examples of all the island’s natural habitats except for the rain forest: coral reefs, turtle grass, sandy and rocky beaches, lagoons, a dry forest, and a mangrove forest. It is home to many endangered wildlife species, including the osprey and the sea turtle, and artifacts of the Igneri Indians, precursors to the Taínos, have been excavated here.
Two main points of interest are found at Las Cabezas de San Juan. One is the neoclassical lighthouse (el faro), built by the Spanish in 1880, making it the island’s second-oldest lighthouse. Today it houses facilities for scientific research in the areas of ecology, marine biology, geology, and archaeology.
The other highlight of Las Cabezas de San Juan is Laguna Grande, a mangrove lagoon filled with microscopic bioluminescent organisms that glow green at night when they sense motion. Several outfitters in the area offer canoe or kayak rides into the lagoon after dark on moonless nights so visitors can witness the biological phenomenon. Swimming in the lagoon is no longer permitted.
Entrance into Las Cabezas de San Juan is by guided tour only. Call for reservations.
To get here, take Carretera 3 to the Conquistador Avenue exit and turn left on Carretera 987. The reserve is on the left after Balneario Seven Seas  recreation area.