You're walking through a forest, admiring the towering trees. But the real action is happening beneath your feet. Under the soil, an ancient internet pulses with life—a network of fungal threads that connects nearly every plant in the forest, trading resources and information in ways scientists are only beginning to understand.

The Hidden Giant Below

The mushrooms you see poking through leaf litter are just tiny fruiting bodies—the apple on a vast underground tree. The real organism lives below: a sprawling network of microscopic threads called hyphae. Each thread measures about one-tenth the diameter of a human hair, yet together they form networks spanning kilometers.

The scale is staggering. The largest known mycelium, a honey mushroom in Oregon, covers almost 10 square kilometers and has lived for thousands of years. It's likely the largest organism on Earth. But size isn't what makes these networks remarkable. It's what they do.

These fungal webs connect to plant roots in a relationship that dates back roughly 500 million years—to when plants first crawled onto land. Without fungi, there would be no forests as we know them. The partnership is simple: fungi can't photosynthesize, so plants feed them sugars. In return, fungi's sprawling networks gather water and nutrients like phosphorus and nitrogen that plant roots can't reach alone.

Scientists call these partnerships mycorrhizae. There are two main types. Ectomycorrhizal fungi wrap around roots in temperate forests, forming sheaths around pines, firs, and oaks. Arbuscular mycorrhizal fungi penetrate root cells in tropical regions, creating tiny tree-like structures called arbuscules. Both types act as extensions of the plant's root system—but they do much more than just forage.

Speaking in Chemicals and Electricity

For decades, scientists assumed these networks were simple plumbing—nutrients in, sugars out. Then Dr. Suzanne Simard started asking uncomfortable questions. In the 1990s, she used radioactive carbon isotopes to track what moved through the networks in Canadian forests. The results shocked the forestry establishment: up to 40% of the carbon in a tree's fine roots came from other trees through the fungal network.

Trees were sharing resources. Not just with their own species, but across species lines. Douglas firs and paper birches, growing side by side, exchanged carbon depending on who needed it most. In summer, when birches had full sun, they sent carbon to shaded firs. In spring and fall, the evergreen firs returned the favor to their leafless neighbors.

But how do trees and fungi communicate to coordinate these exchanges? Fungi are chemical polyglots. They release and respond to dozens of different compounds that float through air and flow through water. When a tree gets attacked by insects, it sends chemical alarm signals through the network. Neighboring trees receive the message and ramp up production of defensive compounds—before the pests even arrive.

Fungi also speak electricity. In 2022, researcher Andrew Adamatzky discovered that fungi transmit electrical spikes through their networks, similar to neurons firing in a brain. These spikes varied wildly—from 1 to 21 hours in duration, with amplitudes between 0.03 and 2.1 millivolts. The electrical signals appear to be byproducts of calcium waves that ripple through fungal cells, coordinating responses across the entire organism.

The comparison to brains is tempting. Mycelia have branched structures like neurons. They reinforce useful pathways and prune inefficient ones. They even use some of the same amino acids for transmitting information. But fungi don't have nervous systems. What they have might be even more interesting: a distributed intelligence with no central control.

The Princeton Revolution

In February 2025, a team of 28 researchers published findings that fundamentally changed how we understand fungal networks. They'd spent three years watching fungi with specially designed robotic microscopes, tracking approximately 100,000 fluid flow trajectories through the networks. The data collection alone would have taken decades by hand.

What they discovered was extraordinary. Nutrient-rich fluids move in two directions simultaneously within the same microscopic tubes. Sugars and fats flow one way while phosphorus and nitrogen flow the other. It's like a two-lane highway operating inside a tunnel thinner than a hair.

The fungi actively manage this traffic. When demand increases near plant roots, the network widens those pathways and increases flow—like opening extra lanes during rush hour. Growing tips act as explorers, pushing into new territory and pulling behind them a massive network of intricate, lacelike mycelium dense enough to forage effectively for phosphorus.

When these exploring tips encounter another branch of the same fungal network, they simply fuse. No wasted effort building redundant infrastructure. It's an elegant strategy for avoiding overbuilding, achieved without any central planning. The fungi leverage purely local conditions to make decisions. Each part of the network responds to its immediate environment, and sophisticated global behavior emerges from millions of local interactions.

Perhaps most surprisingly, the fungi favor future opportunities over short-term gains. They invest resources in pathfinding tips that explore new territory rather than maximizing extraction from known sources. It's a strategy that would make venture capitalists proud.

Talking to Relatives

The networks don't treat all trees equally. Douglas firs can distinguish their kin from unrelated trees of the same species. Mother trees—the largest, oldest trees in a forest—send more carbon through the network to their own offspring than to strangers' seedlings.

This kin recognition suggests fungi might be reading chemical signatures and adjusting their services accordingly. Or perhaps mother trees pay the fungi extra to deliver more resources to specific recipients. We don't know yet. The fungal network isn't a passive telephone line. It's an active participant with its own interests.

The fungi also interpret context. A chemical signal doesn't have fixed meaning. Fungi assess what it means based on other chemicals present, the source, and current conditions. This is sophisticated information processing—understanding that the same word can mean different things in different sentences.

Even more poignantly, dying trees dump their remaining resources into the network. Surrounding trees absorb this final gift. Scientists have started calling it a "last will and testament." Whether this is intentional generosity or just passive leakage remains debated, but the effect is clear: the forest recycles resources from the dying to the living.

What This Means for Us

These discoveries aren't just fascinating biology. They're forcing us to rethink forest management. Clear-cutting doesn't just remove trees—it destroys the underground networks that help forests recover. Soil loses structure. Seedlings lose their support system. Recovery takes decades longer than it should.

Leaving mother trees standing when logging an area maintains the network. Young trees can plug into the existing infrastructure, dramatically speeding regeneration. Some forestry operations are already adapting these practices.

The networks also store massive amounts of carbon—about 13 billion tons of carbon dioxide annually, more than one-third of global fossil fuel emissions. Protecting forests means protecting not just trees but the fungal networks beneath them.

Agriculture might benefit too. Most crops form mycorrhizal relationships, though industrial farming practices often damage fungal networks. Researchers are exploring whether inoculating fields with beneficial fungi could reduce fertilizer and pesticide needs while improving yields.

The cooperative behavior between different tree species also suggests mixed forests may be more resilient than monocultures. When trees can share resources across species lines during stress, the whole forest becomes more stable.

The Forest Thinks

We're still in the early days of understanding these networks. Every study raises new questions. Do fungi actively broker deals between plants? Can trees really recognize their relatives, or are we anthropomorphizing chemical gradients? How much actual information passes through the networks versus simple resource flows?

What's clear is that forests aren't collections of individual trees competing for resources. They're superorganisms—interconnected communities where cooperation is as important as competition. The fungal networks are the connective tissue that makes this possible.

Next time you walk through woods, remember: you're standing on top of an ancient internet. Beneath your feet, millions of microscopic threads are pulsing with nutrients, chemicals, and electrical signals. Trees are talking to each other. Fungi are making routing decisions. Resources are flowing to where they're needed most.

The forest is thinking. Not with a brain, but with a distributed intelligence that's been solving problems for half a billion years. We're only just learning to listen.