With a surface area larger than the British Isles, the most extensive reef system in the world is 2,000km long: the only living structure visible from the International Space Station.
It's made up of more than 3,000 separate fringing reefs that lie between Australia's Queensland coast and the edge of the continental shelf. All exist thanks to colonies of tiny polyps, the "architects" of the reef, whose outer skeletons form intricate multi-coloured coral metropolises.
Monty Halls' recent BBC series showcased this Unesco World Heritage-listed treasure, which is home to not only the world's greatest concentration of coral (400 types of hard coral and 300 types of soft coral) and 1,500 or so species of glittering fish, but also 4,000 breeds of molluscs; 500 types of seaweed; 1,500 varieties of sponges; 200 bird species; and more than 30 marine mammal species including humpback, minke, killer and pilot whales.
The reef stretches from just south of the tropic of Capricorn to north of Cape York, and incorporates about 900 coral islands. For a close-up of the kaleidoscope of corals and marine life teeming below the surface, diving and snorkelling offer a truly hypnotic, outer space-like experience in the warm waters (often as shallow as a swimming pool). The reef lies 16km to 160km off the Queensland shore.
Day cruises generally incorporate about three hours of underwater exploration and include snorkelling gear, with many day trips offering diving as an optional add-on for certified divers.
For a more immersive stay, there is a multitude of overnight or "live-aboard" packages on offer, some incorporating learn-to-dive courses for the as-yet uninitiated.
Reef trips of all kinds run year round, but peak in the winter dry season (May to September); things are quieter during the summer wet season (October/November to March/April), when tropical downpours descend and potentially fatal jellyfish inhabit the waters, which mean swimmers must wear a body-stocking-like "stinger suit".
Accommodation on the islands and along the coast ranges from backpacker hostels to five-star hotels and some gorgeous national park camping grounds, managed by the state's Department of Environment and Resource management (derm.qld.gov.au). For more information, visit Queensland's tourist board website: tq.com.au.
As if one World Heritage area weren't enough, the Great Barrier Reef neighbours another Unesco-listed wonder. The Wet Tropics World Heritage Area covers an area bigger than Wales running north from Townsville, and includes some of the world's oldest rainforest. Numerous tour firms offer reef and rainforest combination deals. Cairns Heliscenic (cairns-heliscenic.com.au) offers hour-long helicopter flights over the city, hinterland and reef.
This fragile ecosystem is under threat from climate change and pollutants. The authorities have responded by creating the Great Barrier Reef Coast Marine Park (www.gbrmpa.gov.au)encompassing 3,600km of coastline.
Gateway to the reef
The provincial sugar cane town turned international resort city of Cairns is the main launching pad for Great Barrier Reef explorations.
Unwitting visitors are often caught off guard by Cairns' mudflats location. (That means no beach.) But the city makes up for it with a sparkling man-made lagoon, some great art galleries and an excellent Aboriginal cultural centre, Tjapukai (tjapukai.com.au) on Kamerunga Road, Smithfield. There's also a sizzling Asian-influenced dining scene, vibrant Night Markets (nightmarkets.com.au) on the Esplanade, and bars and clubs that pulse until it's time to board your reef cruise the next morning.
A mind-boggling 600-plus tours depart from Cairns daily, including numerous learn-to-dive courses. The inner reef areas can be patchy, showing signs of damage.