At the bottom of a gold mine in South Africa, a kilometre-and-a-half underground, scientists have discovered a thriving ecosystem, including tiny animals descended from ancestors, trapped down there thousands of years ago. The unexpected complexity and richness of life in this hostile environment raises new interest in how and where life might persist on other worlds.

Plus - why cuttlefish hold their breath underwater; and how an anti-Alzheimer's gene may have led to the rise of grandparents

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