CHEMISTRY Microlightning between bubbles containing air and methane. Solving the mystery of will-o’-the-wisps Faint blue flames, known as will-o’-the-wisps or ignis fatuus, have been observed over marshes, cemeteries, and wetlands for centuries. The phenomenon has been attributed to the low-temperature oxidation of methane within swamp gas produced by decaying matter. However, how methane spontaneously ignites at ambient temperatures remains unclear. Yu Xia et al.