The largest living organism on our planet is neither animal nor plant. It lives in or, more accurately, under a forest in Oregon, spread over nine square kilometres. It glows eerily in the dark, and scientists reckon it’s about 2,500 years old.

This is not the elevator pitch for a Hollywood sci-fi blockbuster, though I admit the thought has crossed my mind many times over the past year. No, it’s just one of the less bizarre facts that sprung up as my friend and producer Richard Ward researched a documentary for BBC Radio 4 on the strange world of fungi.

Fungi are hot in the science community right now. A vast new frontier that makes the space race look like a school project. Even at the edges of the story, fungi make us question our notions of self, species, sex and community. It’s not the hallucinogenic qualities of mushrooms that open the doors of perception, it’s pretty much everything about them.

My journey started with cooking. Like everyone else, I’m trying to eat less meat. As a result, I’m increasingly obsessed with the potential of mushrooms. In the UK, we’re aware of the possible variety on offer but perhaps not fully convinced. Some north and eastern European nations are mycophiles with thousands of amateur mushroom hunters taking to the forest at weekends and cheerfully feeding their haul to their families. Here, we buy the simplest field mushrooms, plastic-packed from the supermarket, and most would be extremely nervous of trying anything else. In recent years we’ve expanded our repertoire a little, taking in Italian porcini and one or two of the friendlier Asian fungi – the shiitake and the eryngii have become uncontroversial – but if the French are going to insist on calling something edible a “trompette de la mort”, well, it’s not exactly inviting, is it?

It’s easy to see where our squeamish British mycophobia might have sprung up. We associate fungi with death and decay. A tiny minority of fungi are exceptionally good to eat, the majority are dull or actively unpleasant and a small number can cause an agonising death.

Of the edible fungi, it’s perhaps the truffle that is our furthest stretch. It looks like a leathery rock or possibly a particularly coherent lump of dirt, and we still can’t cultivate it with any success. If you asked any group of normal, non-food-nerds to describe its smell, they’d say it was redolent of an unhygienic gym-bag, and they’d be appalled at the thought of putting it in their mouth. Yet truffles are some of the most costly foodstuffs, by weight, in which humans trade — a trade that often involves smuggling, violence, organised crime and even murder. In his recent book The Truffle Underground: A Tale of Mystery, Mayhem and Manipulation in the Shadowy Market of the World’s Most Expensive Fungus, US journalist Ryan Jacobs has uncovered a complex web of deceit in which truffles from eastern Europe — considered less premium than the French or Italian varieties — are routinely smuggled and “re-badged” as the expensive stuff. There have been allegations of strong-arm tactics, including armed raids on storage facilities and corruption at high levels of local government.

In the 1930s there was great excitement around hallucinogens, which indigenous cultures had used since prehistory. “Magic” mushrooms containing psilocybin and LSD — originally derived from ergot, a fungus that grows on rye — were seen by some to have real promise in the burgeoning field of psychotherapy. In recordings of scientists experimenting with the drugs, the complete lack of fear in their voices is remarkable. There’s no sense that they’re doing something dangerous or illegal, just a spirit of excitement and intellectual curiosity. It’s strange to hear the clipped tones of a tweed-clad academic psychiatrist talking calmly about drinking the “decoction” and describing, in wonder, the opening of his own Doors of Perception. Perhaps my favourite find was tapes of Cary Grant, an early and earnest adopter of psychotherapy, talking without guilt about his positively heroic consumption of hallucinogens.

Much has been written about what happened next. How the drugs were tested in university labs, initially by medics but later under the aegis of the CIA, which was quite understandably interested in their potential uses as an interrogation tool or a weapon. How the drugs leaked out into the counterculture and redefined a generation, and the massive state backlash of controlled substances acts in almost every country in the world. Research effectively froze beneath a global media spiral that morphed into a catastrophic “war on drugs”. It’s only in the past decade that the panic seems to have subsided. Psilocybin is now being tested as a therapy for drug dependence, anxiety and mood disorders.

When we started our research, I think we felt that was about as weird as it was going to get. We couldn’t have been more wrong.

Perhaps because of their literary or visual or artistic antecedents — fairies prancing round them or Alice being invited to eat from one by a caterpillar smoking a hookah — we tend to think of fungi in terms of the mushrooms or toadstools we see sticking up through the leaf mulch. But that’s not what fungi are all about. Those are just the fruiting parts of the organism that lives below: a vast web of interconnecting hyphae called mycelium. And the most incredible thing about mycelium is not actually its size, but what it can do.

About a decade ago, I met a couple of young people at a trade show. They’d set up a little tent and were making a very convincing case for growing mycelium around delicate packaged goods, like a natural styrofoam. I’ve no idea if their business took off, but a few years later in a hip east London restaurant, the chef proudly told me that the little button stool I was sitting on was a solid block of mycelium that had been grown into a mould. Then, last year, my daughter took me to a lecture on the future of sustainable architecture, where they blithely discussed skyscrapers, taller and stronger than today’s tedious erections because they wouldn’t be “built” of reinforced concrete but “grown” of mycelium.

If I’m honest, I started to think the damn stuff was stalking me.

In the 1960s, Rank Hovis McDougall identified Fusarium venenatum as a fungal mycelium suitable for human consumption. It’s amazing stuff. You fill a tank with a nourishing growing medium, inoculate it with spores, feed it plenty of oxygen and the network of tightly packed hyphae doubles in size every five hours. If you’re a fan of Quorn, the well-known edible mycoprotein, then this is great news. If you’re a fan of 1950s science fiction movies, you might have reservations.

Quorn is an uncomplicated fungal material that provides texture and dietary protein. It isn’t designed to replicate meat, but today, as commercial interest in meat replacements becomes ever more fevered, many researchers are looking to fungi. It seems likely that they will continue to feature more and more in processed and manufactured food, and there is no real upper limit to the amount they could eventually contribute to our diets.

The uses of mycelium seem to grow as fast as, well, mycelium. On a freezing cold Thursday in October, I met Dr Sebastian Schornack at the Sainsbury Laboratory Cambridge University (SLCU). Deep underground in a highly secure “biocontainment facility”, he placed the roots of a small plant that had grown in the same substrate as a mycelium on the stage of a microscope. In a secure research environment like this, it’s permissible to create plants with genetic modification and Schornack had grown tiny tobacco plants containing the three genes that together make beetroot purple. They are so arranged that they will combine in the tobacco roots to create a pink colour when they come into contact with the mycelium. Even at quite modest magnification, I could see clearly that the pink material was inside the cells of the plant root and in the mycelium touching it. Schornack is infectiously excited. “It’s the most intimate relationship that you can imagine. The fungus really lives inside each and every plant cell.”

But that’s not the end of his discoveries. “This relationship is more than 400 million years old . . . There are already structures like these fungi inside the cells of plants in the fossils.”

What’s toughest to get your head around here is that fungi have been “part of” plants since the very beginning. Just as we humans are appreciating that we’re hosts to our own living gut ecology, and always have been, we realise that plants and fungi have a similar “relationship”. But that’s an inadequate word; it’s an inseparable coexistence. And if plants have always been “part fungi”, and if we ourselves have always been “part” the fungal and bacterial mix of our gut “flora”, then it makes you question the comfortable notion of discrete kingdoms or organisms.

But now it becomes even more complicated. Dr Suzanne Simard is a professor of forest ecology at the University of British Columbia. During her research, she injected radioactive carbon into a birch tree and, after a while, got a reading on her Geiger counter in a neighbouring Douglas fir. Material from one plant appeared in another entirely different type of plant. Not only was material being transmitted but, by selectively delivering nutrients, the fungus could affect the growth of the trees. Mediated by the mycelium, signs of “distress” in one tree could affect how others grew. They were communicating via a third organism that wasn’t even in the same kingdom. We need to pause here for a second to let the ramifications of that sink in. That’s like a cat transmitting information and having it delivered to a rhino in a way that changes its behaviour . . . by a carrot.

This does not, to be clear, equate to trees “talking to each other” but it does mean that by operating as a medium of transmission, the fungus seems to enable the entire forest to “act” in its own interests, almost like a single organism. The result has been called, by Simard and others, the Wood Wide Web.

That mycelium facilitates and mediates interspecies communication between trees and may, in some way, alter their growth on behalf of their collective wellbeing, to be honest, just blows my conceptual buffers. Tree, fungus, forest? Who’s in charge here? What is the entity?

Under Kew Gardens, away from the crowds, is the biggest collection of preserved fungal specimens in the world, and it was here that the curator Lee Davies told me the macabre story of the zombie cicadas. These giant flying bugs look a bit like locusts and are common all over the central and eastern states of the US. They have a very brief and intensive mating season during which a small percentage will be infected by a fungus called Massospora. This produces a variety of chemicals in their bodies, including psilocybin and an amphetamine called cathinone, a cocktail that provokes frenzied sexual activity in the cicada. Which is unfortunate, because the fungus also colonises the creature’s entire body and causes the back end, including its genitals, to fall off before replacing it with a big white ball of spores. Somehow the fungus also forces them to perform both male and female mating behaviours to attract even more cicadas to a futile fumble that infects them with the spores.

I felt this may have been the most Byzantine form of reproduction I’d ever encountered, which is why I found myself earnestly discussing the sex lives of fungi with the biologist and writer Merlin Sheldrake. “There are so many different ways for fungi to do sex,” he explained, “some of them have tens of thousands of mating types roughly equivalent to our sexes . . . it just seems like there are lots of different ways to go about shuffling your genetics and, as flexible and collaborative and diverse organisms, they’ve found flexible and collaborative and diverse ways to do so.”

I can’t get through a conversation about gender with my teenage daughter without my brain melting. What hope have I got of comprehending the mathematics of fungus sex? Even at our current, very limited level of understanding, fungi force us to ask complicated questions. If a fungus can entirely occupy a living organism, control its behaviour and use its body, we’re going to need a new word to express the thing that’s flying around. At precisely what point does it stop being a possessed bug and become a mobile mushroom? Perhaps more troubling is Sheldrake’s gentle reminder that I am full of, and covered by, all sorts of fungi. I can’t live without them and they can’t survive without me. So am I a single entity or a well-dressed and mobile ecology?

Lichens are some of the oldest living things on our planet. There are some estimates that they cover 7 per cent of the earth’s surface in one form or another, but they’re not plants. Though they photosynthesise, they don’t have roots and draw no nutrients from the surfaces they grow on. Lichens are actually algae, living entirely in the mesh of a fungal mycelium. It would be madly inaccurate to call them “hybrid” and it’s a more than symbiotic relationship. They can’t be separated from each other, and they’ve been living like this for longer than humans or most of the animals and plants we know today. We still don’t really have a name for something like this.

Mycologists are an interesting bunch. Their enthusiasm has kept them focused on what’s long been a neglected backwater of botany. They have seen things we mortals wouldn’t believe and, now that the edges of the veil are lifting, their excitement is justifiably enormous. We met researchers who felt they could change and improve almost every area of human interest with what they were uncovering. Quite aside from the simple nutritional possibilities and the pharmacological potential, there are varieties that can survive extreme conditions, enhance plant growth or kill pests; some types in research at the moment will consume plastics, clear up oil spills and even, potentially, neutralise nuclear waste.

Yet other scientists and activists are alarmed that we don’t understand the damage we’re doing to the fungal world. The Fungi Foundation, an international NGO established by a brilliant Chilean mycologist called Giuliana Furci, articulates this peril. Its aim is to educate and inform about the diversity of fungi and to promote their use as innovative solutions to problems we may have yet to discover. Most importantly, the Fungi Foundation is changing the language of public policy, encouraging the international community to think of the natural world in terms of the three F’s: flora, fauna and “funga”.

The fungus community is as diverse and brilliant a group as you can imagine, but it shares one firm belief: there is so much we don’t know and so much left to discover. Where fungi are concerned, we are only beginning to know how much we don’t know.Sooner or later, everyone who researches fungi experiences a similar weird reaction, somewhere between physical and emotional. Sheldrake calls it “vertigo”, and I think he’s bang on with the observation.

I’d gone out for a hike over some of the very few hills near Cambridge. I sat on a bench and looked out at the surrounding countryside and, suddenly, I felt dizzyingly aware of all of it. I was sitting beneath trees, so it was underneath me. For miles into the distance I could see woods, coppices, separate stands of trees. Under each, maybe between them, unseen but vast masses of mycelium. If they were above ground, they’d be bigger than dinosaurs, taller than buildings. Living organisms of almost inconceivable size. Ancient, slow to grow, slow to move, responsive to their environment. You realise that there is fungus in the air, on the surface of or inside almost every living thing, and the feeling that creeps up on you is, yes, the spinning, disorientating sensation that you’re suddenly not quite so connected to the ground underneath you. It’s vertigo all right.

Some time after the Enlightenment, we started looking at nature differently. We no longer needed to fear that our environment would kill us and we went out — to mountains, to forests, to the sea — to be surrounded by something huge, something infinitely bigger and older than ourselves, to put ourselves in perspective. These days, it may still feel pretty impressive to see a mountain, a rough sea or a stretch of desert, but film, photography and even cheap adventure holidays have robbed us of that sudden, staggering revelation.

There’s a painting by the German Romantic Caspar David Friedrich. The English title is something like “Wanderer above the Sea of Fog”. You’re sure to have seen it. It’s a rear view of a man in an overcoat, standing on the very peak of a mountain and staring out over a landscape of almost incomprehensible scale. You can’t see his face, but go out this weekend, stand on a hillside somewhere, look out at the trees, consider the fungi and you too can feel what he’s feeling.

“Fungi: The New Frontier”, written and presented by Tim Hayward and produced by Richard Ward and Loftus Media for BBC Radio 4, is on at 9pm on April 12 and 19, and also available on BBC Sounds

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