And the answer: Great Barrier Reef.
The Great Barrier Reef is off the coast of Queensland, Australia, in the Coral Sea. It consists of 10 percent of the world's coral reef ecosystems and is so vast it can be seen from space. The Great Barrier Reef is also a marine-protected area, meaning the government has limits in place to shield the marine ecosystem from intrusive human activity.
The Great Barrier Reef is the largest living structure on Earth. Between the Queensland coast and the western edge of the Pacific Ocean, the Great Barrier Reef spans over a thousand miles, and contains over 70 types of habitats. Most of the thousands of species of marine life which call the Barrier Reef home could not thrive in any other place on the planet.
The formation of the Great Barrier Reef began 10,000 years ago, at the end of the last Ice Age. When the ice at the poles melted, floods of water began to overtake areas on Australia's low-lying coast, and coral began to grow in its wake. The shallow tropical waters were clear and warm— perfect conditions for coral to thrive. While much of this coral is living and continues to grow, the basis for much of the reef dates back millions of years. As a result, many of its inhabitants can appear a bit... well, prehistoric. For one, the alien-like nautilus is a distant cousin to the squid, yet this creature has miraculously remained relatively unchanged over the last 500 million years.
Today, the Great Barrier Reef is one of the Seven Natural Wonders of the world and a UNESCO World Heritage Site. Learn more about the history and wildlife in the Great Barrier Reef here.