A Kilometre Across

The climb is twenty minutes on a path of loose grey grit that slides half a step back for every step up, and it gives you no clue at all what’s coming. The outside of Hverfjall is just a black slope. Then you reach the top and the whole thing opens out at once — a circular pit a kilometre wide, walls running down at a constant angle all the way round, and a low mound sitting in the middle of the floor where the vent was.

It is astonishingly regular. It went up in a single explosive eruption when rising magma hit groundwater, roughly two and a half thousand years ago, and dropped everything it threw out in a ring around the hole. Nothing has really disturbed the shape since. There is no vegetation in it, no water in it, and no scale in it either — I could not tell whether the specks on the far rim were people until they moved.

Cold wind straight across the top, and the sound of it going over the rim was the only thing to hear.