How was built ? Coral reefs are made of tiny animals called “polyps” that stay fixed in one place and are the main structure of a reef. Polyps have a hard outer skeleton made of calcium (similar to a snail’s shell). Each polyp is connected by living tissue to form a community. Only the top layer of a coral reef contains living polyps. As new layers of the coral reef are built, the polyps leave the lower layers.Coral reef formation involves a variety of processes operating at different scales, and our discussion of how coral reefs are formed reflects that reality. While a wide variety of marine life ultimately contributes to the structural complexities of coral reefs, most of the reef's underlying solid framework is constructed by just a few types of marine organisms.They are the stony corals, and the coralline red algae. A great variety of other organisms including many types of invertebrate animals assist the two primary types of reef builders. When ? The world's first coral reefs occurred about 500 million years ago. Where ? The coral reef is in the ocean. Coral reefs are found all around the world in tropical and subtropical oceans. They are usually found in shallow areas at a depth of less than 150 feet. However, some coral reefs extend even deeper, up to about 450 feet deep. How ? Hard corals build reefs by growing a top in the stony skeletons of previous coral colonies. What they are ? Of course, the Great Barrier Reef is not the only coral reef on Earth. These living, growing, changing structures thrive in waters within a 30-degree band north and south of the Equator. Coral reefs are rocky mounds or ridges formed in the sea by living things through the accumulation and deposition of limestone (calcium carbonate). These are also the best places to visit if you want to experience marine life. Reefs grow best in warm, shallow, clear, and sunny waters. Often called "rain forests of the sea", coral reefs form some of the most diverse ecosystems on Earth. They occupy less than 0.1% of the world's ocean surface. Coral reefs are found in ranging to depths of 60 millimeters. These oceans are home to more species of fishes, corals, and many other type of marine life than any other ocean habitat. Corals are sessile, which means that they permanently attach themselves to the ocean floor, essentially "taking root" like most plants do. Today, these important habitats are threatened by a range of human activities. Many of the world’s reefs have already been destroyed or severely damaged by water pollution, over fishing and destructive fishing practices, disease, global climate change, and ship groundings. However, we can still protect and preserve our remaining reefs by acting now. Some coral reefs can take thousands of years to form. The coral reef environment has a physical structure that is radically different than that of the surrounding open ocean. Fishes that live in reef habitats have therefore evolved a host of specialized traits well suited to their colorful and complex homes. The coral reef environment has a physical structure that is radically different than that of the surrounding open ocean. How much time has it been in life ? Coral reefs have been around for millions of years. Appearing as solitary forms in the fossil record more than 400 million years ago, corals are extremely ancient animals that evolved into modern reef-building forms over the last 25 million years. What it provides for us ? Coral reefs provide many services to humans. Food, shoreline protection, and medicines are just a few of these benefits. They also protect shorelines from big waves by absorbing wave energy and provide a safe place for fish to spawn (release eggs into the water). Coral reefs provide habitats for a large variety of organisms. They also provide food (fish and shellfish) for many people living along coastlines. They even help in the carbon cycle. How can we help the coral reef ? Walk, bike or ride the bus. Fossil fuel emissions from cars and industry raise lead to ocean warming which causes mass-bleaching of corals and can lead to widespread destruction of reefs. The less water you use, the less runoff and waste water will pollute our oceans. Although you may live thousands of miles from a coral reef ecosystem, trash and other products flow into the water system, pollute the ocean, and can harm coral reefs and marine life.You can also ask the fishing, boating, hotel, aquarium, dive or snorkeling operators how they protect the reef. Be sure they care for the living reef ecosystem and ask if the organization responsible is part of a coral reef ecosystem management effort.You should plant trees.Trees reduce runoff into the oceans. You will also contribute to reversing the warming of our planet and the rising temperatures of our oceans.