The fissure of Silfra opened in 1789 due to the movements of the tectonic plates that frame Thingvellir National Park.Some of the ravines fill with groundwater traveling underground through the porous lava fields in the area, originating from Langjokull glacier about 37 miles (60 kilometers) north. It can take the water up to a century to reach Silfra, and this long filtration process results in the water being both extremely clear and drinkable.