"Keriš Crater". Keriš is a 6,500 year old volcano crater located in the Grimnses region of Iceland. It is one of a line of several craters, known as Tjarnaholar within the ancient Grimsneshraun lava field. The lake is approximately 890' x 560' and its depth varies up to 50', depending on the fluctuation of the underground water table. The red-ish rocky soil is from iron oxide, thrown up by the volcano.