High-speed cameras have captured the moment when sparks initiate the phenomenon known as ignis fatuus or will-o’-the-wisps, finally offering an explanation for a phenomenon that has terrified and delighted humanity for millennia. The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

The bobbing blue lights seen in wetlands have formed the basis for a great many of humanity’s stories . Would our folklore of elves and sprites be nearly as rich without them? Four centuries ago, theologian Ludwig Lavater noted that the phenomenon mostly occurs in places where organic material rotting in the absence of oxygen forms methane, mixed with small amounts of other molecules to form “swamp gas”. In the late 18 th century, scientists produced evidence

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