Cosmopolitan Port Douglas, in the tropical far north of Australia, is the ideal place to embark on a dive tour of the planet's largest natural structure.

And if you are the sort of person who enjoys learning all about the marine environment you can join a pretty unusual style of dive voyage - one with an extra element of discovery. We had booked onto Undersea Explorer, a liveaboard that focusses on the understanding and conservation of the Great Barrier Reef.

Eco-tourism is the buzzword and there's always a marine biologist on board who takes time to explain all about the 3000 individual reefs, 1500 species of fish, 400 corals and 4000 molluscs. Our itinerary took us along the Ribbon Reefs, the fringing reefs that wind down the coast and define the outer edge of the continental shelf before it drops off into the deep water, Queensland trench.

The Ribbons are characterised by two styles of dive - gently sloping rim reefs and more dramatic pinnacle dives. These tend to be tall, thin rocks, like an inverted chimney, that soar from around 30 metres until they almost break the surface. There's usually a mass of snappers, large schools of jacks, tuna and mackerel around them. Smaller guys include shrimps in mushroom corals or anemones, whip coral gobies, nudibranchs, leaf fish and the usual tropical fish.

In the winter months (June & July) dwarf minke whales are a frequent visitor. You can snorkel with them and if you are really lucky, they may visit during a dive.