Coral reefs are major structures, which are not attributable to human beings on our planet.
The largest is the Great Coral Reef, in Australia, over 2,200 kilometres long.
They extend approximately for 600,000 square kilometres, across all tropical seas where temperatures are not lower than 17 degrees.
The cliffs, better defined by the English term reef, are built by microscopic animals, polyps, .... (less info)