Anne Pringle was surveying mushrooms at a field site in Tomales Bay State Park, just north of San Francisco, when she found herself in a predicament. She was surrounded by a sea of one of the world’s most dangerous mushrooms: Amanita phalloides, commonly known as the death cap.

“I couldn’t put my foot down without stepping on them,” Pringle says. “It was just a valley of death. A total infestation.”

That was 20 years ago, when Pringle, now a mycologist at the University of Wisconsin–Madison, was doing research at the University of California, Berkeley. Despite its proliferation, there was a rumor that the deadly mushroom hadn’t originated on the Golden Coast. Six years and much DNA sequencing later, Pringle proved the rumor true: North America’s death cap mushroom was an invader, a fungal species likely native to Europe.

Now found thousands of miles outside that original range, death caps are the culprit behind most mushroom-related poisonings. Their powerful toxins start to attack the human body in as little as six hours after they’re consumed, causing abdominal pain, nausea, and vomiting that, if untreated, can result in fatal liver failure. Last August three people in Australia died from ingesting death caps, victims of an alleged poisoning. The mushroom—about five inches tall with a greenish yellow-white cap—can be easily mistaken as edible. In British Columbia, a child died after eating one in 2016; in Northern California, 14 people fell severely ill in 10 separate incidents during one particularly scary week in 2017.

When a fungus is moved anywhere from dozens to thousands of miles away, these complex relationships can go haywire.

But death caps didn’t evolve to kill people. These mushrooms are mycorrhizal fungi. They spring from a tangle of fungal threads that grow in soil and curl around tree roots, helping the trees take up nutrients. This activity underfoot both intrigues and worries scientists, like Pringle, who say we know too little about the fungal kingdom and what happens when these underground networks are rewired.

Over the past century, our world has become more connected than ever, and fungi, like the death cap, have embarked on countless global journeys, hitching a ride on imported plants or simply wafting hundreds of miles in the wind. Now climate change is allowing many of these organisms to thrive in ecosystems that were once too cold and dry. If history is any indication, we may not be ready for what’s in store.

Though fulgens is Latin for “dazzling,” these orange-hued mushrooms (Caloscypha fulgens), found here on Greece’s Mount Olympus, are a pathogenic fungus, killing and mummifying the dormant seeds of conifers.

In a sense, fungi are a hidden earthly dimension we’re only now learning how to see.

They thrive in soil and grow edible stalks like plants, but many of their characteristics are distinctly unplantlike. Where plants have cell walls made of cellulose, fungi have chitin, a type of fiber also found in the exoskeletons of insects and crustaceans. And fungi are heterotrophs—capable of eating other organisms, often breaking down wood and dead plant matter by releasing and reabsorbing enzymes. Without fungi, dead plants and animals would pile up on forest floors, and most trees would struggle to find the nutrients they need to survive.

“They’re probably closer to animals than you think,” says Rabern Simmons, the curator of fungi at the Purdue University Herbaria.

For more than a billion years, fungi have evolved to live in specific environments, sometimes in partnership with just one other species. But when a fungus is moved anywhere from dozens to thousands of miles away, these complex relationships can go haywire. “It’s a perfect storm with fungal pathogens,” says Stephen Parnell, an epidemiologist at the University of Warwick who models the spread of plant disease.

Diverse strategies for reproduction help a fungus survive. Airborne spores from different species can intermingle in a new habitat, or the mushrooms might fuse together the threads that form their underground networks. But in a pinch, many can simply reproduce asexually.

With climates and landscapes changing at record pace, says Parnell, these reproductive traits make fungi uniquely—and worryingly—adaptable. In new environments, foreign fungi can spread voraciously and remake the topography around them.

American chestnut trees were once giants of Appalachia, growing a hundred feet tall and 10 feet wide. In the early 20th century, however, the fungus Cryphonectria parasitica landed on American soil. In Japan and China, the fungus was only a nuisance to Asian chestnut trees, but for the American chestnut it caused deep cankers that slowly choked it of water and nutrients. An estimated four billion trees died over the following century as a result.

As the last great American chestnut trees withered, frogs and other amphibians faced a similar peril with a fungal pathogen known as chytrid. Believed to have originated on the Korean Peninsula, the fungus lived in harmony with local amphibians. But over the past 150 years, chytrid has spread around the globe and is now associated with the decline of at least 500 amphibian species; it’s caused 90 species to disappear from their habitats. It’s been described as the worst wildlife disease in history.

“We’re moving biological material across the world in a matter of hours, across continents that were long separated,” says Ben Scheele, an ecologist at the Australian National University. “We essentially have re-created dysfunctional Pangaea.”

The aptly nicknamed coral spine fungus (Hericium clathroides) cascades from a tree in the Chalkidikí  Peninsula of Greece. 

Under the galaxy purple cap of this toxic beauty, the Laccaria amethystina mushroom accumulates high levels of arsenic.

Last autumn, Pringle and one of her students spent weeks collecting hundreds of death cap mushrooms from golden-hued forests in the United Kingdom, Hungary, France, and Poland. These samples could help scientists better understand why death caps thrive in some ecosystems and don’t in others.

Researchers are looking for a predator or pathogen they can replicate to stop the mushrooms from invading forest floors, a method called biocontrol. But Pringle says one of the most effective ways to keep fungi in the right environment is prevention: Monitor imports of foreign species, and test them for fungi.

When fungal diseases can’t be prevented in an environment, treating them can become an immense undertaking. To restore the American chestnut, several scientists have been working on decades-long breeding projects, one of which involves a controversial genetically modified tree. And while individual frogs can be cured of chytrid, eradicating the fungus in environments where it’s introduced is nearly impossible. Last year, new research on how death caps produce their powerful toxin opened the door to a possible antidote.

Porcelain mushrooms (Oudemansiella mucida), like these “towering” from a beech tree  on Mount Olympus, reach between one and three inches tall.

Some start-ups and nonprofits have promising solutions for helping fungi help us. Funga, a company in Austin, Texas, identifies native fungi that can assist trees in storing more carbon. SPUN, a scientific research organization, is mapping the world’s fungi to identify regional hot spots in need of conservation. At least 350 species are already at risk of going extinct, though the real figure is likely much higher.

For Simmons at Purdue’s Herbaria, winning the race against the biodiversity clock is critical—for humanity and fungi alike. “We’re finding things that are beneficial to humanity in some way, whether it be the production of compounds like biofuels or compounds that are understood to have medicinal purposes.”

What’s kept protected, he adds, may one day solve the next problem we might create.