We can’t hear them, but the trees are whispering. Below our very feet and at a microscopic level that might be hard to fathom, trees chatter and gossip in the soil even as they appear still on the surface.

It’s in this subterranean world that trees can warn each other of imminent dangers, care for sick neighbors, and send each other food. In 1997, an August issue of Nature headlined “The wood-wide web,” announced the finding that tree species in the wild can talk back and forth with each other. This became big news, and inspired scientists to further explore this communication web.

Millions of microbes are found in soil, and, much like the human microbiome, can affect—positively and negatively—the health of the ecosystem. Nearly all land plants—around 90%—have long-standing, symbiotic relationships with fungi. Trees found in temperate regions tend to associate with what are called ectomycorrhizal fungi for their day-to-day functions. By wrapping themselves around the roots of the trees, these fungi have a rather intimate relationship with their hosts. And, in many cases, the fungi connect the roots of neighboring plants to form common mycorrhizal networks. Like bundled cable fibers, these networks allow neighboring plants of the same or of different species to chit-chat away. One study, published in 2009, described a fungal network that wove its way through an entire forest, over distances as great as 20 meters.

All these secret signals occurring underground are similar to the ways plants communicate above-ground, too. Plants secrete chemicals in the air to warn their neighbors of imminent threats and dangers. So even if a tree falls in a forest and no human is around, it still makes a sound. The fungi are listening—they always have, and always will.