It was already very ancient; its foundation had been laid by Roman soldiery; in the lapse of ages much of it had sunk, and every here and there, for a few hundred yards, it lay submerged below the stagnant waters of the fen.
-- Robert Louis Stevenson, The Black Arrow: A Tale of the Two Roses, 1888
We could do nothing but get drenched while the earth spat mud around us and the fen steamed and boiled.
-- Jeremy Page, Salt, 2007
Fen came from the Old English term fenn, and ultimately comes from the Proto-Indo-European root pen- meaning "swamp."