1. Great Blue Hole, Belize
When seen from above, the Great Blue Hole looks like the pupil of an eye. Seen from within, this Unesco World Heritage-listed ocean sinkhole is a visual treat for divers. Ringed by fringing reef, and approximately 400m in diameter, the Great Blue Hole drops away to around 145m. About 40m down are the formations that lure divers from around the world: marine stalactites up to 15m in length. Marine life is noticeable only in its absence - you might not see a single fish - but when you're swimming among stalactites, who gives a Nemo? Day trips depart at 6am and return at 5.30pm, or you can spend the night aboard the boat. Watch out for the sea serpent, sighted in the 1960s.
2. Chuuk Lagoon, Micronesia
Micronesia's Chuuk Lagoon is rich in colourful coral and tropical fish, but for divers these are almost peripheral to the main attraction. What draws 56 divers to this 70km-wide lagoon are the wrecks - Chuuk may hold the greatest proliferation of shipwrecks in the world. A Japanese naval base in WWII, here dozens of ships were sunk and many planes downed during US attacks in 1944. Dives include the Fujikawa Maru, complete with intact fighter planes in its holds, and the Shinkoku Maru, decorated by nature with soft corals and sponges. Only permit holders can dive; arrangements can be made at the Blue Lagoon Dive Shop on the island. Visit www.bluelagoondiveresort.com.
3. Manta Ray Village, Hawaii
No prizes for guessing the star attraction at this dive site off the Kona coast of Hawai'i (the Big Island), though half the fun is that dives here are conducted at night. Dive operators shine powerful lights into the water to attract plankton, which in turn attract manta rays (which then attract divers). Manta-ray sightings are unreliable - you might see up to 10 rays and their magnificent 'wings', or you might see none. Dives during the new moon seem to be the best bet for manta encounters. You can opt for the three-hour round-trip snorkel or do a certified one-tank manta-ray night dive. Book through www.hawaiiactivities.com.
4. Samarai Island, Papua New Guinea
Get down and dirty in the world's muck-diving capital as you swim through Samarai's silty waters to appreciate the finer things of the sea. You won't encounter whale sharks, manta rays or moray eels on this island off Papua New Guinea's southeastern tip; Samarai is about the little critters, such as nudibranches. Shallow waters make Samarai's tiny ocean goodies accessible even to novices, and you'll find exuberant corals and tropical fish, as well as remnants of the island's turbulent history. Check online or with operators at Milne Bay; www.telitadive.com has a good reputation. Luxury full-berth cabins are US$300-340 per person, per night.
5. Pulau Sipadan, Malaysia
Slow things down to turtle pace as you take to the seas off the Malaysian island that invariably figures in all lists of the world's top dive sites, Pulau Sipadan. Green and hawksbill turtles abound; there's even a so-called turtle tomb, 22m underwater, containing the skeletal remains of vast numbers of turtles. For a marine adrenalin rush, try Barracuda Point, where the eponymous barracuda often gather in swirling, tornado-like formations. No diver will want to leave without witnessing the famous Drop Off where, just a stroll from the shore, the ocean floor drops away 600m. Night diving typically costs around MYR150 per dive for one to three divers, or MYR50 if there are four or more. Book early as permits are restricted to 120 per day.
6. Cocos Island, Costa Rica
On Cocos Island, 600km off Costa Rica's Pacific coast, it's hammer time. Some of Jurassic Park's most evocative scenes were filmed on this island, but it's under the sea that things are truly wild. Here, hammerhead sharks shoal in enormous numbers, off ering divers a jittery look at their fantastic features. The largest shoals are found around the submerged mountain at Alcyone, where you will also see white-tip reef sharks and possibly whale sharks. Divers will need to visit on liveaboard boats as nobody is allowed to stay on the island. November to May (dry season) means calmer seas, silky sharks and large schools of mobula rays. Rainy season (June to November) equals large schools of hammerheads but rougher seas.
7. Gansbaai, South Africa
Move up the food chain, from hammerheads to great white sharks, as you climb inside a metal cage and come nose to snout with the ocean's most fearsome predator. Watch in awe, even as you wonder about the cage's strength, while the 6m-long great whites circle. Dive operators off this Western Cape town use bait to attract the sharks to the cage, virtually guaranteeing sightings (and controversy). You'll find operators based in Hermanus, though the boats leave from Gansbaai, 35km away. Cage diving costs around ZAR1100 per person. Transport from Cape Town is offered by most operators and personalised DVDs/videos are usually available too.
8. Ras Mohammed National Park, Egypt
Covering the southern tip of the Sinai Peninsula, this national park is the final landfall before the underwater wonders of the Red Sea. The park itself contains 20 dive sites, many of them among the Red Sea's finest. Two submerged peaks, Yolanda Reef and Shark Reef, are the park's diving centrepieces, and both are rich in marine life. At Yolanda you can look forward to diving among the wreckage of the Yolanda, including its cargo of hundreds of toilet bowls (and a BMW). The vertical wall at Shark Reef is prized for its concentration of fish and, unsurprisingly, sharks. All visitors must leave the park by sunset. Snorkellers and divers standing on the corals devastate the reefs so please be careful.
9. Cocklebiddy Cave, Australia
Australia's Nullarbor Plain may appear waterless, but beneath this enormous limestone block there's a series of caves, including Cocklebiddy Cave. This 6.7km-long, arrow-straight tunnel is almost entirely flooded, making for one of the world's premier cave dives. It was here in 1983 that French cavers racked up the world's longest cave dive by exploring to Cocklebiddy's end. The cave is situated 10km north of remote Cocklebiddy Roadhouse; divers must obtain permits from Western Australia's Conservation and Land Management (CALM) department. Experienced cave divers only; no tours are offered. Cocklebiddy's Wedgetail Inn, a caravan or rooms for AU$50-150 a night, is about your only accommodation option.
10. Rainbow Warrior, New Zealand
Bombed by French government saboteurs in Auckland harbour in July 1985, the Greenpeace boat Rainbow Warrior was later refloated and scuttled off beautiful Matauri Bay in New Zealand's Northland. Coated in colourful corals and populated by goatfish, moray eels and other fish, the Rainbow Warrior sits upright in 25m of water, wedged into the sandy ocean floor. Anemones, sponges and algae of all colours cling to the wreck; in its grave the Rainbow Warrior is far more rainbow than warrior. Book a day tour through www.divehqboi.co.nz or visit www.divetours.co.nz to book a seven- or 15-day tour that includes a number of dive sites.