Hverfjall

Where it sits. Hverfjall rises east of Lake Mývatn in northeast Iceland as an almost perfect ring of black tephra, about a kilometre across at the rim and 140 metres deep inside. The rim stands roughly 420 metres above sea level and about 140 metres above the lava plain at its foot, which puts the whole cone within an hour's walk of the Route 860 car park. It dominates the eastern side of the lake basin and shows in the background of most photographs taken at Dimmuborgir, Grjótagjá and Höfði. Locals argue about the name: Hverfjall and Hverfell are both in use, the dispute is old, and both appear on maps and signage in the district.

How it formed, and how fast. Hverfjall is a tuff ring, built in a single short phreatomagmatic eruption roughly 2,500 years ago. Rising basaltic magma met groundwater in the porous lava beneath the site, the water flashed to steam, and the resulting explosions shredded the magma into fine ash and coarse tephra rather than letting it flow as lava. The debris was thrown out ballistically and piled into a symmetrical rampart around the vent within days or weeks, not years. The interior is a bowl of loose black gravel with a low mound near the centre. Because the material is unconsolidated and the slopes sit close to their angle of repose, the cone erodes readily, and every footprint off the path becomes a rill that rain then deepens.

Why the path rules are strict. Two routes are permitted, and only two. The northwestern path climbs from the main car park on the Route 863 spur in about twenty to thirty minutes of steep, loose walking, gaining roughly 140 metres. The southern path descends toward the Dimmuborgir side and is the usual way down for anyone making a traverse. Everything else is closed, including the tempting direct lines up the western face, because the scars from earlier decades of free walking are still visible from the lake road. A rim circuit is about 3.2 kilometres on a narrow, exposed track with a drop into the crater on one side; it is easy underfoot in dry conditions and unpleasant in high wind, which this ridge attracts. Descending into the crater bowl itself is not permitted.

On screen. Hverfjall does not carry a named Game of Thrones scene of its own, and honest reporting matters more here than a sales line. What it does carry is the skyline. The wildling camp sequences filmed at Dimmuborgir in late 2012 look out on this cone, and the wider plates of lava field shot around Mývatn for the third season include its unmistakable dark ring on the horizon. Visitors following the series around the north will pass it whether or not they intend to, and the rim gives the single best overview of the whole filming district: Dimmuborgir below to the southwest, Grjótagjá's fissure line to the north, the lake beyond, and on a clear day the white edge of Vatnajökull far to the south.

What you actually see today. The climb is short but genuinely steep, on gravel that slides underfoot, and it is a poor choice for anyone unsteady on loose ground. The reward at the rim is a view that explains the geology of the district better than any interpretation panel: the pseudocrater fields on the southern shore of Mývatn, the lava that dammed the lake, the steaming ground at Bjarnarflag, and Krafla's ridges to the north. Inside, the crater is silent and windless in a way the rim never is. The tephra is a uniform charcoal black, so the cone photographs best with snow lying in the gullies or with low sun raking across it in the early morning or late evening.

Getting there and timing. The turn-off is about five minutes by car from Reykjahlíð and two minutes from Grjótagjá; the last stretch is gravel and usually fine in a normal car in summer. There is no fee, no shelter and no water, and the car park has limited space that fills with coach traffic in the middle of the day. Weather changes the calculation more than season does: the rim is fully exposed, and the gusts that funnel across the lake basin can make the narrow track genuinely dangerous. In winter the paths hold snow and ice, spikes are essential, and the descent is harder than the ascent. Allow ninety minutes for the climb and rim circuit, or forty-five if only going up and back.