Perhaps you’ve heard of something called a “coral reef.” These are large underwater structures that provide a habitat for up to 25% of all known ocean species. But there’s something even more remarkable about coral reefs. The coral itself is alive!

Corals are formed by groups of small animals called polyps. Most corals get their energy, nutrients, and color from algae. In exchange, the coral provides a safe space for the algae to live. This is called symbiosis. The algae are also the reason for the coral’s color.

Some of the world’s most famous coral reefs are the Great Barrier Reef near Australia and the Bikini Atoll. The Great Barrier Reef is the largest in the world, stretching over 2300 kilometers (1400 miles). The Bikini Atoll is about 3800 kilometers (2400 miles) to the northeast of this reef. An atoll is a ring-shaped coral reef that surrounds a sunken island. Bikini swimwear was named after the Bikini Atoll.

Rising water temperatures and pollution are a threat to coral reefs. When the water becomes too hot or too dirty, the algae leave. As a result, the corals turn white and die. This is called bleaching. The Great Barrier Reef has lost more than half of its coral this way since 1985. Only by working to stop pollution and climate change can we ensure that the reef will stay alive for future generations.