fen fire

fen fire (plural fen fires)

1. will o' the wisp
   • 1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887, →OCLC:

     Also we were above the level of the marsh mist, which lay stretched beneath us like the dim smoke-pall over a city, lit up here and there by the wandering globes of fen fire.