The subtle glow that can appear on a forest floor after rain

On a wet, dark evening, step into the woods, switch off your torch and give the darkness a moment. A faint green gleam may gradually emerge from a fallen branch, while tiny mushroom caps can look like pinpricks of light among the leaves.

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.