A 31-square mile (50 sq. km) crater that is still volcanically active. There are many craters and lakes in this area. It’s a great place to hike through. The lake Öskjuvatn has an active vent near the northeastern corner, which exploded in the 1875 eruption and created the crater Viti. Today, the crater contains a warm lake and swimming in this 77 F (25 C) warm water is a treat you shouldn’t miss when you’re there. So bring a swimsuit. Askja is located north of Vatnajökull.