The Border Mire is the bog north of Alkai Tor, fed by two freshwater rivers; its western and central reaches are freshwater marsh and peat, while the eastern half becomes a toxic saltwater marsh from natural underground salt deposits. The terrain is unstable—soft peat, quickmud, reed beds, and tidal flats—where poisoned springs, hidden sinkholes, and marsh predators (quaggoths, kuo-toa, burrowing beasts) make travel hazardous. Locals know to follow raised ridgelines, carry ropes and iron stakes, and purify water before drinking.