3-mile hike up to and around a massive tephra explosion crater - a full km wide and 460 feet deep. It was a fairly steep trail, with an elevation gain of about 700 feet. It is dark and has a beautiful almost perfectly circular shape, like a stadium or an amphitheater, with a large mound in the middle.From the rim, we could look out towards lovely Lake Myvatn on one side, and the ubiquitous steam rising from the earth on the other.