To astronauts who've seen it from space, the Great Barrier Reef resembles a vast, snaking wall—like a moat running parallel to Australia's entire northeastern coast. Up close, what looks like a barrier is in fact a labyrinthine complex with millions of points of entry. Mind-boggling in size and scope, encompassing more than 4,000 separate reefs, cays, and islands, the Reef could rightly be called its own subaqeous country.
The Great Barrier Reef is a living animal.
However, it's hard to imagine that the reef, which covers an area about half the size of Texas, is so fragile that even human sweat can cause damage. Despite its size, the reef is a finely balanced ecosystem sustaining billions of tiny polyps, which have been building on top of each other for thousands of years. So industrious are these critters that the reef is more than 1,640 feet thick in some places. The polyps are also fussy about their living conditions and only survive in clear, salty water around 18°C (64°F) and less than 98 feet deep.
An undersea enthusiast could spend a lifetime exploring this terrain—which ranges from dizzying chasms to sepulchral coral caves, and from lush underwater "gardens" to sandy sun-dappled shallows—without ever mapping all its resident wonders. Not only is the Reef system home to thousands upon thousands of sea-life species, the populations are changing all the time.
The Great Barrier Reef begins south of the tropic of Capricorn around Gladstone and ends in the Torres Strait below Papua New Guinea, making it about 2,000 km (1,240 miles) long and 356,000 square km (137,452 square miles) in area.
Apart from the coral, divers can swim with more than 2,000 species of fish, dolphins, dugongs, sea urchins, and turtles.
Great Barrier Reef, Queensland, Australia