The largest structure ever built by living organisms is visible from space

Australia's Great Barrier Reef stretches about 2,300 kilometres and covers roughly 344,400 square kilometres, built from nearly 3,000 individual reefs accumulated from the skeletons of billions of tiny coral polyps over thousands of years. It's the only living structure on Earth large enough to be seen from orbit with the naked eye. Rising ocean temperatures have triggered repeated mass coral bleaching events across the reef in recent decades.

— Wikipedia contributors, Great Barrier Reef — UNESCO World Heritage Site