The Great Barrier Reef, much like the Earth it's situated on, can be seen from space, though probably not with the naked eye. By that logic though, almost anything can be seen from space.
There are over 2,900 individual reefs located in the Great Barrier Reef, which is located on the coast of Queensland (pronounce the 'a', dammit) on the east coast of Australia. It attracts large of numbers of tourists, which Australians love, because they pump their foreign currency into our businesses and backpacker lodges.
A popular past time in the Reef is scuba-diving, where you can see some of the most amazing and beautiful shit that anyone has ever seen. Ever. There's a minimal chance you will be eaten or munched on like an afternoon snack, provided you stay in the right places and don't get left behind by your tour boat (that happened once - there's a movie about it.)