Yesterday we visited one of the natural wonders of the world - the Great Barrier Reef. Everything about the reef is larger than life. It is the biggest structure built by living organisms on the planet. It stretches for 2,300 km along the Queensland coast and can be seen from outer space. It is home to approximately:

• 1,500 species of fish
• 360 species of hard coral
• 1/3 of the world's soft corals
• 4,000 species of molluscs
• 800 species of echinoderm (starfish, sea urchins etc.)
• More than 30 species of marine mammals including whales, dolphins and dugongs
• 6 of the 7 species of marine turtle
• 14 species of sea snake, and
• 200 species of bird.

Many of these are threatened and not found anywhere else.

The reef is only about 8,000 years old and, despite its size, is built entirely from very small polyps. These grow a limestone case. It is colonies of these polyps which makes up the structure of the coral. Some of the corals can be several metres in size. The polyps have a symbiotic relationship with algae called zooxanthellae. The algae grow in the limestone tube and use sunlight and the polyp's waste products to make oxygen and food. These, in turn, provide the polyp's food. Because the algae need sunlight corals are only found in relatively shallow water.

Getting to the reef involves a long boat trip. From Port Douglas we travelled about 70 km to the Agincourt Reefs. Today the sea is quite rough and several people are sea-sick (to spare their blushes I shall not name names!) Then we have to get into less than flattering lycra 'stinger suits'. There are several potential nasties on the reef including Irukandji and Box Jellyfish, Blue Ringed Octopuses, Cone Shells and Stone Fish, and the suits provide protection against some of these. Then it's on with our flippers and face masks and jump into the water for some snorkeling.

Up close the reef is pretty spectacular. Some of the corals are brightly coloured - blue, purple, yellow. There are hundreds of fish ranging from the tiny clown fish ('Nemo' - thanks to Tahn, one of the crew, for the photograph) - no more than a few centimeters long - up to Maori Wrasse which can reach 2 meters. Many of these are also brightly coloured and all seem oblivious to the human swimmers thrashing around about them. Down in the murky depths - although the water is incredibly clear - I spot a large Reef Shark.

All too soon we have to get back on the boat for the return and equally rough, trip back to Port Douglas. It's been a fantastic day.