Beneath our feet is an ecosystem so astonishing that it tests the limits of our imagination. It's as diverse as a rainforest or a coral reef. We depend on it for 99% of our food, yet we scarcely know it. Soil. Under one square metre of undisturbed ground in the Earth's mid-latitudes ... there might live several hundred thousand small animals. One gram of this soil less than a teaspoonful " contains around a kilometre of fungal filaments. But even more arresting than soil's diversity and abundance is the question of what it actually is. Most people see it as a dull mass of ground-up rock and dead plants. But it turns out to be a biological structure, built by living creatures to secure their survival, like a wasps' nest or a beaver dam. Microbes make cements out of carbon, with which they stick mineral particles together, creating pores and passages through which water, oxygen and nutrients pass. The tiny clumps they build become the blocks the animals in the soil use to construct bigger labyrinths. Plants release into the soil between 11% and 40% of all the sugars they make through photosynthesis. They don't leak them accidentally. They deliberately pump them into the ground. These complex chemicals are pumped into the zone immediately surrounding the plant's roots, which is called the rhizosphere. They are released to create and manage its relationships. The rhizosphere lies outside the plant, but it functions as if it were part of the whole. It could be seen as the plant's external gut.