Rising from the ocean floor like thousand-story skyscrapers, wall reefs are fed by nutrient-rich currents that traverse their steep vertical terrain. In turn, wall reefs become the anchoring foundation for giant barrel and tube sponges, huge gorgonian sea fans, magnificent fields of alveoli-shaped soft corals, and of course, marauding schools of pelagics (including a litany of oceanic predators such as sharks, rays and barracudas).

Regardless of their origin, vertical wall reefs produce many of diving's most memorable and thrilling adventures. To novice divers, vertical wall diving is the big step taking off the water wings and plunging into the pool's deep end, which seemingly has no bottom. Staring down into a vast, bottomless space can have quite a dizzying effect. Slipping over the edge of the wall is like launching into space and seeing the edge of the earth slip off into oblivion. But with proper dive planning, good buoyancy control and attention to space (time and depth, too), vertical wall diving is quite safe and can be habit-forming.

Wall reefs such as Little Cayman's world-renowned Bloody Bay Wall, and the wall in front of CoCo View Resort on Roatan Island, are easily accessible and festooned with marine life (barrel and tube sponges, lobsters in crevices and parades of fish). For the most part, they are usually void of strong currents, so divers of almost any level can feel the thrill of slipping over the edge. The lush shallow reefs (30 feet or less in depth) that meet their drop-offs also provide divers with wonderful subaquatic real estate to spend time off-gasing (instead of hanging on a deco-bar). Vertical wall diving in these locations can evoke a real sense of peace and tranquility.

Conversely, vertical walls such as Fiji's Great White Wall (off Taveuni Island) and the Red Sea's Elphinstone Reef feature high-octane currents. These walls produce tree-size soft corals, chorus lines of golden anthias and visits from opportunistic predators. Swimming next to the wall is optional. Going with the flow is often much wiser.

Throughout all oceans, vertical wall reefs are a spectacle to behold and a wonder to dive. They provide shelter and nourishment for a mind-boggling array of creatures, and an underwater mountain for divers to both scale and soar over like an eagle surveying its world.

Bloody Bay Wall Arguably the best vertical wall dive in the Caribbean, Bloody Bay Wall is also the Cayman Islands most prolific marine park. Located on the northwest coast of Little Cayman, this huge limestone cliff rises from depths of 7,000 feet to within 20 feet of the surface. Giant barrel sponges and tube sponges the size of organ pipes propagate the steep slopes. Turtles and spotted eagle rays are frequent fliers along the drop-off.

Roatan The Cayman trench stretches west to Roatan, creating some fantastic wall diving opportunities along the island's north coast. Colorful sponges and interesting invertebrates abound, silversides pack swim-throughs, and if you are lucky, a whale shark may pass your way.

Deku Dekura This wall in the Solomon Islands begins with a straight drop of 100 feet through an ultra-blue limestone cavern, which curves up slightly and empties out onto spectacular vertical wall reefs. Clownfish and tiny tridacna clams hang out on the wall as occasional sharks, turtles and manta rays swim by.

Elphinstone An elongated barrier reef 900 feet long and 200 feet wide, Elphinstone has all the components of a great wall dive: deep and steep walls, soft corals coming out of every hole and cave, and the chance to see big pelagics such as manta rays, dolphins, hammerheads and oceanic whitetip sharks. With steep walls, little wind protection and strong currents, Elphinstone, like other mythic Red Sea dives, is an experienced diver's Mecca.

Grand Turk Wall A short distance offshore of the island bearing its name, Grand Turk Wall is a pristine vertical wall reef. Beginning at an average depth of 35 feet, the wall drops dramatically into a deep blue abyss. Visibility often exceeds 100 feet, offering extraordinary underwater panoramas and frequent sightings of large pelagics. The reef topography is peppered with numerous crevices and ledges where marine life abounds in a kaleidoscope of color.

Molokini Crater A classic volcano-created vertical wall, Molokini Crater is a cone-shaped formation that just breaks Hawaii's Pacific waters. Accessible by boat Molokini offers shallow dives on the inside and vertical walls on the outside. At times, large manta rays are seen visiting the inner realm, along with many species of resident fish and whitetip sharks. The outside wall is often visited by large pelagic animals.

Peleliu In 1944, Peleliu Island was the site of one of World War II's bloodiest battles. Today, beneath the waves of infamous Bloody Nose Ridge, this southern Palau island welcomes more civil guests to enjoy its vertical wall reef. Brilliant red gorgonian sea fans, harboring longnose hawkfish and various invertebrate crawlers, hang from the wall, while pretentious lionfish inhabit hard-coral lairs. Frequent sightings of color-blushing cuttlefish are also reported.

Salt River Canyon When sea levels rose after the last ice age, it turned a valley on St. Croix's northern shore into a facing pair of vertical walls. Today, divers flock to the twin slopes of Salt River Canyon's East and West Walls to experience the only true wall diving in the waters of the Virgin Islands.