James' World 2

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Directly beneath our feet stretches a world both smaller and much, much larger than the one we know. Scientists have just begun mapping what may well be the largest piece of infrastructure on Earth: a network of fungal threads that is 68 quadrillion miles long.

It’s alive!

And we thought we’d mapped just about everything there is to map on this planet: continents, voting patterns, bird migration routes, alleged apparitions of the Virgin Mary, and Starbucks locations near you.

We’ve logged and named almost every fold of the visible world. And yet until June, the most extensive piece of infrastructure on the planet had not been extensively mapped. This network is larger than every road, cable, and shipping lane combined. And it’s alive.

To be precise: It’s fungal. This underground network is made of so-called hyphae: hair-thin filaments that knit together into vast structures known as mycelium. A large fraction of these fungi form partnerships with the roots of plants and trees. Roughly 70% of all botanical species indulge in these relationships.

Think of these relationships, also known as mycorrhizae, not so much as tender friendships, but as shrewd trade agreements: Both parties get something indispensable out of it. The plant can make sugar from sunlight, but is bad at foraging. So the fungus provides phosphorus, nitrogen, and water. The fungus is a foraging superhero, but cannot photosynthesize. So the plant repays the fungus in carbon.

How to survive on land

The arrangement is not new — it dates back several hundred million years, to when plants were first figuring out how to survive on land. What’s new is that scientists have now learned to map the actual shape and extent of that arrangement.

Writing in Science in June, a team including members of the Society for the Protection of Underground Networks (SPUN), estimated that the top 15 centimeters of the planet’s soils hold roughly 110 quadrillion kilometers of so-called arbuscular mycorrhizal fungal hyphae.

That distance in miles sounds no less deranged: 68 quadrillion. But to get some sense of the vastness of that funny-sounding number, imagine laying those filaments end to end, in which case they would stretch from the Earth to the Sun close to a billion times. No use: that’s still an absurdly large mental object to wrap your head around.

Another way of gauging the network’s size is to consider its weight instead of its length. In total, it is estimated to weigh around 300 million tons, which, the authors write, is roughly five times the biomass of all humans on Earth combined.

How could we have missed such a massive network? Well, it’s also extremely tiny — a teaspoon of soil can hold up to 10 meters of fungal hyphae, each fragment so thin as to be practically invisible.

4 billion tons of CO2

To map something this humungous and diaphanous, researchers had to develop a new mapping discipline — a cartography of dirt. They collected and analyzed 16,000 soil samples from all over the world, extrapolating this map from the data they gathered.

Easy to lose behind those quadrillion miles of living filaments is another big number. The fungal network moves around four billion tons of carbon dioxide equivalent into the soil each year. That is equivalent to 11% of humanity’s annual emissions. For a structure that few people knew about until a few months ago, the global fungal network is doing good work on a planetary scale.

That talent for burying CO2 may be why we will finally pay attention to this subterranean layer of life, beginning with this eerily glowing map of what has already been called the “fungal internet,” published by SPUN.

Although still incomplete, the map already offers some surprises. If you had to guess where Earth’s fungal network was densest, “rainforests” sounds like a reasonable answer: lushly layered flora, thick with roots. But no. The map points to grasslands as the hotbed of fungal filaments — as much as 40% of the world’s arbuscular mycorrhizal network is buried beneath the waving fields of grass of the Tibetan Plateau, and the flooded grasslands of the Everglades and South Sudan, among other places.

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This global underground network is invisible to the naked eye, yet is more than five times the weight of all humans on Earth. (Credit: SPUN/A Hidden Infrastructure

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