Way beneath every step you’ve ever taken and every step you will take is this thing called mycelium. Long threads of fungi that grow one cell at a time, branching and re-branching, growing in every direction. They even grow three-dimensionally.

These fungi are mutualistic. They connect the trees with the soil in a market exchange of carbon and nutrients and link the roots of paper birches and Douglas firs in a busy, cooperative Internet. When the interwoven birches and firs were spiked with stable and radioactive isotopes, I could see, using mass spectrometers and scintillation counters,

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