Stretching more than 1,400 miles along Australia's northeast coast, the Great Barrier Reef holds a remarkable distinction: it's the largest living structure on Earth, and one of the few biological features visible from space.

This underwater metropolis isn't a single continuous reef, but rather a complex system of 2,900 individual reefs and 900 islands sprawling across 133,000 square miles. That's roughly the size of Italy, or larger than the United Kingdom.

A City Built by Billions

The reef's architects are coral polyps—tiny animals no bigger than a grain of rice. Over millions of years, these microscopic creatures have collectively constructed the world's largest living structure, secreting calcium carbonate to build their external skeletons. When they die, their skeletons remain, and new polyps build on top, layer upon layer.

The result? A structure so massive that astronauts can spot it from the International Space Station, 250 miles above Earth.

An Underwater Universe

The reef's scale creates one of the planet's most diverse ecosystems:

1,500 species of fish call it home
400 types of coral form its foundation
4,000 species of mollusks inhabit its crevices
Six of the world's seven sea turtle species breed here
Humpback whales migrate through its waters annually

Age and Fragility

The current reef structure is relatively young—around 6,000 to 8,000 years old—though reef systems have existed in the region for over 20 million years. But this ancient wonder faces modern threats.

Rising ocean temperatures have triggered mass coral bleaching events, where stressed corals expel the algae they depend on for food, turning ghostly white. The reef has lost half its coral cover since 1995, with warming waters, pollution, and crown-of-thorns starfish outbreaks taking their toll.

Despite conservation efforts, scientists warn that without aggressive climate action, the reef's future remains uncertain. What took millions of years to build could fundamentally transform within our lifetimes.

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