Science
Why the forest floor seems to glow after rain
Some fungi give off a faint green light, and a damp, moonless night is the best time to notice it. All you need is patience and a torch you are willing to switch off.
Walk into a damp forest on a dark night, turn off your light and wait. At first there is nothing. Then, slowly, a soft green shimmer begins to show along a fallen log, or a cluster of small caps appears in the leaf litter like a handful of tiny lamps.
Light without heat
The glow is called bioluminescence. A number of fungi produce it through a chemical reaction inside their cells, part of the same broad family of reactions that lets fireflies flash. Unlike a firefly, though, the fungi do not blink. Their light is steady and faint, and it often comes from the fine threads running through rotting wood as much as from the mushrooms themselves.
People have noticed it for centuries. In English it has long been known as foxfire, a name that suits a light you only ever catch out of the corner of your eye.
Why some fungi glow is still being explored. One idea is that the light draws in insects that help carry spores around; another is that it is simply a by-product of the fungi’s chemistry. For now, the forest is keeping some of its secrets.
How to see it
Rain helps: the glow tends to be easier to spot when wood and soil are wet. Choose a night with little or no moon, keep away from street lights and give your eyes plenty of time to adjust, longer than you might expect. If you need light to move around, a dim red one is gentler on your night vision.
Look, but do not pick. Glowing fungi are part of the slow work of turning fallen wood back into soil, and some mushrooms are poisonous. Stay on the path, go with someone who knows the area, and leave everything as you found it.
Then switch off the torch and wait. The forest is brighter than it looks.