Over the past few years, a fascinating narrative about forests and fungi has captured the public imagination. It holds that the roots of neighboring trees can be connected by fungal filaments, forming massive underground networks that can span entire forests — a so-called wood-wide web. Through this web, the story goes, trees share carbon, water, and other nutrients, and even send chemical warnings of dangers such as insect attacks. The narrative — recounted in books, podcasts, TV series, documentaries, and news articles — has prompted some experts to rethink not only forest management but the relationships between self-interest and altruism in human society.

But is any of it true?

The three of us have studied forest fungi for our whole careers, and even we were surprised by some of the more extraordinary claims surfacing in the media about the wood-wide web. Thinking we had missed something, we thoroughly reviewed 26 field studies, including several of our own, that looked at the role fungal networks play in resource transfer in forests. What we found shows how easily confirmation bias, unchecked claims, and credulous news reporting can, over time, distort research findings beyond recognition. It should serve as a cautionary tale for scientists and journalists alike.

First, let’s be clear: Fungi do grow inside and on tree roots, forming a symbiosis called a mycorrhiza, or fungus-root. Mycorrhizae are essential for the normal growth of trees. Among other things, the fungi can take up from the soil, and transfer to the tree, nutrients that roots could not otherwise access. In return, fungi receive from the roots sugars they need to grow.

As fungal filaments spread out through forest soil, they will often, at least temporarily, physically connect the roots of two neighboring trees. The resulting system of interconnected tree roots is called a common mycorrhizal network, or CMN.