What is the largest living structure on Earth?

The Great Barrier Reef runs some 2,300 kilometres along the Queensland coast, covers an area larger than Italy, and is visible from orbit. It is built by coral polyps, each a soft animal a few millimetres across that secretes a calcium carbonate skeleton beneath itself; the reef is the accumulated skeletons of countless generations, with a thin living layer on top.

The polyps do not manage it alone. Inside their tissues live microscopic algae that photosynthesise and hand over most of the sugar they make, which is what allows a reef to be so productive in tropical water that is otherwise nearly barren. Coral colour comes largely from those algae.

That partnership is also the vulnerability. When the water is too warm for too long the coral expels its algae, loses its colour, and begins to starve - which is what bleaching is. The reef has suffered mass bleaching events with increasing frequency since 1998, several in the last decade, and the intervals between them are now shorter than the time a damaged reef needs to recover. There is a rival for the title, incidentally: a single stand of quaking aspen in Utah called Pando is one organism connected underground, and by mass it beats any reef.