Out of the Blue: It is the only World Heritage Site visible from outer space, and indeed the statistics that describe the Great Barrier Reef verge on the astronomical. This coral wilderness spans 135,135 square miles, encompasses about 2,900 reefs, and has greater diversity than any other ecosystem on the planet. How best to explore a site so vast? Here, eight ways to take the plunge

Where to…

Swim with the whales
Cod Hole, in the Ribbon Reefs, is famous for its resident population of potato cod (named for their large and lumpy heads) and its winter visitors—dwarf minke whales, who make the annual trek here from colder climes every June and July. Top trips, including those with Undersea Explorer, involve scientists and snorkeling (61-7-4099-5911; undersea.com.au; seven-day tour, $2,363).

Sleep under the stars
Rugged, remote Lizard Island has 24 spectacular beaches, easy access to prime dive spots, and the swank Voyages Lizard Island resort, where all rooms have patios kitted out with daybeds or hammocks (61-2-8296-8010; lizardisland.com.au; doubles, $1,440, all-inclusive). The more affordable but no less star-studded option is the campsite on Watsons Bay, on the island's northwest side (epa.qld.gov.au; camping fee, $4 per person).

Keep your feet dry
Agincourt Reef is perfect for non-swimmers who want to spot sealife. From Port Douglas, join a cruise boat with underwater observatories, get a fish-eye view in a semi-submersible, or suit up in a large Oceanwalker helmet and stroll along a reef platform. Quicksilver Cruises offers all three options (61-7-4087-2100; quicksilver-cruises.com; day cruises, $110–$170).

Get romantic
Lie low and live large at the Orpheus Island Resort, a 60-minute puddle jump from Cairns; there are no TVs, phones, or children under 15. While the island—all 3,380 acres of which is a national park—is unspoiled, guests are not. The latest treat is a seven-course tasting menu on the island's private jetty (61-7-4777-7377; orpheus.com.au; doubles, $1,320–$1,545, all-inclusive).

Wreck dive
The coral-encrusted steamship S.S. Yongala sank about 55 miles southeast of Townsville in 1911. Today, it is a biological wonder awash in everything from giant Queensland gropers and sea snakes to eagle rays and barracuda. Book a trip with Yongala Dive (61-7-4783-1519; yongaladive.com.au; one-day dives, $195).

Sail a tall ship
Overnight sailing trips around the Whitsunday Islands feature visits to secluded coves. Book through Tall Ship Adventures (61-7-4946-7124; tallshipadventures.com.au; three-day cruises, from $426).

Eat, drink, and watch the boats come in
Hamilton Island, the most accessible of the Whitsundays, hosts the Great Barrier Feast, June's three-day gourmet food festival (61-2-9433-0444; hamiltonisland.com.au), and August's Race Week, Australia's largest offshore yachting regatta (hiyc.org.au). Qualia is one of the Coral Sea's most luxurious new resorts (61-2-9433-3349; qualia.com.au; doubles, $2,543–$2,900, all-inclusive).

Coo at baby turtles
The reef is home to six of the seven extant species of marine turtle. From January through May, hatchlings burst from eggs and flap their way from beach to sea on the tiny resort island of Heron, a haven for snorkelers and birders (61-2-8296-8010; heronisland.com; doubles, $336). Hatchlings are also found at Lady Elliot Island, a resort at the southern end of the Barrier Reef (61-7-5536-3644; ladyelliot.com.au; doubles, $263–$450).

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When to go
June through October, to avoid the wet season's intense humidity.