A will-o’-the-wisp is a ghostly light seen by travellers at night, especially over bogs, swamps or marshes. It resembles a flickering lamp and is said to recede if approached, drawing travellers from the safe paths. A folk belief well attested in English folklore and in much of European folklore, the phenomenon is known by a variety of names, including jack-o’-lantern, hinkypunk, hobby lantern in English.
Scientifically, “marsh gas” is methane that bubbles out of marshes; this gas is contaminated with phosphine (PH3) and diphosphane(P2H4) which, when brought in contact with air, can spontaneously catch fire. This sudden burst of flame can potentially explain many will-o’-the-wisp sightings.
In European folklore, these lights are held to be either mischievous spirits of the dead, or other supernatural beings or spirits such as fairies, attempting to lead travellers astray.
Danes, Finns, Swedes, Estonians, and Latvians amongst some other groups believed that a will-o’-the-wisp marked the location of a treasure deep in ground or water, which could be taken only when the fire was there. Sometimes magical tricks, and even dead man’s hand, were required as well, to uncover the treasure. In Finland and other northern countries it was believed that early autumn was the best time to search for will-o’-the-wisps and treasures below them. It was believed that when someone hid treasure, in the ground, he made the treasure available only at the midsummer, and set will-o’-the-wisp to mark the exact place and time so that he could come to take the treasure back. Finns also believed that the creature guarding the treasure, aarni, used fire (aarnivalkea) to clean precious metals.
The will-o’-the-wisp can be found in numerous folk tales around the United Kingdom, and is often a malicious character in the stories. In Welsh folklore, it is said that the light is “fairy fire” held in the hand of a púca, or pwca, a small goblin-like fairy that mischievously leads lone travelers off the beaten path at night. As the traveler follows the púca through the marsh or bog, the fire is extinguished, leaving the man lost. The púca is said to be one of the Tylwyth Teg, or fairy family. In Wales the light predicts a funeral that will take place soon in the locality.