The site was unlike anything I had ever seen in the Caribbean. Rising from a depth of 90 feet to within 15 feet of the surface, the ridges, blunt buttresses and immense boulders of Yellow Reef reminded me of the volcanic pinnacles of the Eastern Pacific. But I was not in the Pacific, I was diving Desecheo Island, off Puerto Rico's northwestern corner.Dropping below 50 feet, I encountered a series of large cave openings that led deep into the reef. Dive lights revealed a rich mosaic of red, pink, orange, purple, blue and yellow hues. Encrusting sponges, clusters of red and pink coralline algae and purple lace corals formed a living skin that covered every square inch of the tunnel walls.It was not what I expected.Actually, I didn't know what to expect on my first dive trip to Puerto Rico. I knew that the island was one of the top American vacation destinations in the Caribbean, with colorful colonial towns, white-sand beaches and resorts that ranged from quaint to extravagant. As I soon discovered, the island offers a similar variety of underwater adventures, and location is key to both the quality and nature of the diving experience.The island's northern shore is battered by Atlantic swells and the typically low-relief coral reefs are often subject to surge and reduced visibility. Confine your diving to the area around San Juan and you'll miss the best that Puerto Rico has to offer.Travel east and you will find a world of coral gardens and mini-walls set against a sand bottom that rarely drops below 70 feet. Sites on this expansive shoal, which extends eastward to the Virgin Islands, are influenced by the mixing of Atlantic and Caribbean currents and by seasonal rainfall. Visibility can range from 20 to 100-plus feet, and reef life is abundant.Fajardo's protected bay provides predictably calm conditions, while Humacao offers more exciting underwater terrain riddled with canyons, caves and tunnels.The offshore islands of Vieques and Culebra also offer mild surface conditions and relatively shallow sites. In addition, minimal rainfall and the lack of rivers make for consistently clear water.More seasoned divers should head for the southwestern corner of the island to experience the current-washed walls of Parguera. This 22-mile-long stretch of reef features excellent visibility and lush sponge and coral growth, including large trees of black coral. A short distance from shore, shallow spur-and-groove coral structures transition to sheer vertical faces beginning at depths of 60 to 80 feet. Schools of reef fish hang in the current and pelagic hunters cruise in from the blue depths. This is wall diving at its best and most exciting. On the island's northwest corner, the town of Aguadilla provides the island's best opportunity for shore diving on patch reefs and piers. Here, I also visited a B-29 bomber lost in a flight training exercise in the 1940s. The aircraft's massive radial engines were covered in encrusting corals and sponges, but remained attached to the 100-foot wing. For divers willing to brave the challenging sea conditions of the Mona Passage, the islands of Desecheo and Mona offer some of the most spectacular diving in the Caribbean. Home to more than 250 species of fish and some of the healthiest coral growth in the region, Mona also straddles the migration route of whales, dolphin and gamefish on the move between the Atlantic and Caribbean. Desecheo, with its colorful fringing reefs and massive underwater pinnacles, was unlike anything I had encountered in more than two decades of diving in the Caribbean. Truly, it was more than I expected.