John pulled the diving mask over his face and gave the boat's captain the thumbs-up.
He held on tightly to his underwater camera, leaned back, and let his own weight carry him overboard.
He began to descend to one of the earth's greatest natural wonders: the Great Australian Barrier Reef.
Slowly John's eyes adjusted to the reduced level of light underwater.
He was amazed by what he saw.
Large groups of brightly colored fish swam past.
One group was a brilliant yellow; the next striped with orange and black lines.
Each group was like a wonderful moving painting of amazing colors.
Everywhere he looked, he saw a different kind of marvelous fish, a different expression of nature's beauty.
Then the reef came into view. It looked like an underwater city.
Most of the reef seemed to be covered with green algae.
All around, however, the colorful animals and plants that clung to and swam near the coral reef stood out in sharp contrast.
Blue starfish crawled slowly over the reef.
Strange multicolored fish called flatworms performed an underwater ballet as they swam above it.
Overwhelmed by the beauty of the reef, minutes passed before John remembered that he had his camera; then, quickly, he began to take pictures.
Australia's coral reef, at two thousand kilometers long, is the largest in the world and the one most visited by divers.
But coral reefs are not well-understood.
Many believe coral is a type of plant. It's not.
Corals are small animals that eat the tiniest shellfish.
Most corals swim through the ocean, but others called hard corals make a permanent home for themselves.
Hard corals take carbon and phosphates out of the water to make limestone.
As the limestone structure where they live builds up, it becomes a coral reef.
Each individual coral is known as a coral polyp and can be as small as five millimeters across.
It takes millions of coral polyps, living side by side, to make a coral reef.
Each generation of hard corals builds on the remains of its ancestors, and over time, the reefs grow.
But coral reefs would not be possible without the help of coral algae, a type of plant that lives with corals.
Coral algae produce the phosphates and carbon that the hard corals use to make limestone.
The limestone gives the algae a perfect home, safe from most predators.
In return, the algae help the corals to speed up their production of limestone.
Coral algae, just like any other plant, need sunlight to live, and so coral reefs are almost always fifty meters or less from the surface of the ocean.
The algae and the corals both contribute to building the coral reef.
When two different types of life live together and help one another, we say they have a symbiotic relationship.
Working together, coral polyps and coral algae create a coral reef.
But the coral reef is more than just a collection of corals and algae; thousands of species of animals and plants make coral reefs their home.
Some scientists estimate that over 25 percent of all ocean life lives in or near a coral reef.
Many other animals that live near coral reefs also enjoy symbiotic relationships.
There are anemones that will sting, paralyze, and eat any fish that comes nearby.
But these anemones make an exception for the clown fish.
Clown fish help keep the tentacles of the anemones clean, while the anemones provide the clown fish with a safe place to live and breed.
They also share meals. Both animals eat fish, and each enjoys the leftovers that the other creature leaves behind.
The coral reef is also home to both hunter and prey.
The parrot fish can take huge bites out of the coral reef, digesting both corals and algae.
The fish spits out the limestone. Perhaps the oddest predator of the reef is the crown-of-thorns starfish.
It is strange and dangerous. It slowly moves over the coral reef digesting everything in its path.
Unlike most starfish, it has more than 5 limbs (typically between 12 and 20),
and is usually 25 to 30 centimeters wide, but some can grow up to 80 centimeters.
It is bright red and has spines that can inject a poison that would make an unfortunate diver sick for days.
Some fish use the coral reef to hide from predators.
But predators hide inside the coral reef as well.
The moray eel will suddenly leap out from its hiding place to eat a passing fish.
The octopus, too, can hide in the reef.
It changes color like a chameleon, so that it looks like the surrounding coral reef.
It can squeeze itself into almost any opening and wait for prey to approach.
A crab might be crawling near the reef, only to be captured by the octopus' eight tentacles and quickly eaten.
After taking many photographs, John surfaced. Every photograph of a coral reef is valuable, for there may come a time when coral reefs are extinct.
Coral polyps and coral algae are sensitive to temperature, and global warming is heating the ocean, making it harder for reefs to grow.
Some divers chip off pieces of the reefs to take home.
Some people poison the water near the reefs in order to collect the beautiful fish and sell them for aquariums.
The poison only paralyzes the fish, but it kills the corals.
Even worse, some fishermen use dynamite near coral reefs.
The fish are killed and float to the surface, while huge parts of the coral reef are damaged.
Many coral reefs are shrinking; they are being damaged faster than they can grow.
John hopes that more people learn to appreciate the beauty and importance of coral reefs.
Perhaps then, he thinks, more people will work to save the reefs,
and future generations will always be able to visit the marvelous underwater world of the ocean's coral reefs.