A series of mucky lowlands, the fen, or water-meadow, is home to water-loving plants that decay to form peat. Unlike a bog, which has acid soils, the fen is alkaline or neutral, and often occurs above a limestone bed. The name comes from the fenlands of eastern England, now mostly drained for agriculture. America’s most famous fen is—or was—in Boston. Frederick Law Olmsted converted this “noxious tidal swamp” into part of his Emerald Necklace of parks, but the name remains attached, among other things, to the Hub’s beloved ballyard, Fenway Park.