The limestone here formed during the Carboniferous Period, when warm, tropical water habitats dominated. Subsequent weathering formed the karst landscape we see today. Lough Mask flows into Lough Corrib (and then the sea) via underground rivers and springs. Pigeon Hole Cave is one of these conduits that carries the water underground between the two loughs, and can be seen on the walking trail through Cong Woods. In terms of water flow, Cong Spring is 5th largest in the world. The dried-up Cong Canal, a man-made channel, is further evidence of the porous nature of the limestone here.