A pale blue light flickers in a dark marsh. Do you follow it or run? This episode explores the will-o’-the-wisp, the atmospheric ghost light documented on nearly every continent, starting with the surprisingly mundane meaning of the name, the invented Latin term ignis fatuus, and the wicked wanderers of British and Irish folklore such as Will the Smith and Stingy Jack, whose turnip lantern became the jack-o’-lantern.

From there it travels the globe, from the souls of unbaptized children in France and Germany to Brazil’s eye-eating fiery serpent, the Boitata, the Chinese character for ghost fire on Shang dynasty oracle bones, and the treasure lights of Paraguay and Finland. Then it turns to the science: Major Blesson’s 1832 night hunts in German and Polish swamps, methane and phosphine chemistry, cold flames, micro-lightning between bubbles, piezoelectric quartz, barn owls, and why the dark-adapted human eye may have turned green flames blue.

• Why wisp just meant a bundle of burning sticks, and how a 16th-century humanist coined ignis fatuus
• Predatory lights of the Americas: the Boitata, Louisiana’s feu follet, and Trinidad’s soucouyant
• Why humans link ghostly lights with buried treasure, and the psychology of guarded places
• Blesson holding his breath to ignite paper on a ghost light, and the oily residue that matched phosphine chemistry
• Why sightings have nearly vanished as the world’s swamps and fens were drained