Dimmuborgir

Where it sits. Dimmuborgir, the dark castles, is a field of lava pillars, arches and hollow towers on the eastern shore of Lake Mývatn in northeast Iceland, signposted off Route 848 about ten minutes south of Reykjahlíð. It covers roughly two square kilometres of broken ground between the lake and the tephra cone of Hverfjall, which rises directly to the north and shows in almost every wide photograph taken here. The formations stand from a couple of metres to well over ten, many of them hollow, some pierced by openings large enough to walk through. Birch scrub, angelica and crowberry have colonised the sheltered hollows, so the site is far greener at ground level than its reputation suggests.

How the rock got that shape. The pillars are the residue of a lava lake. Around 2,300 years ago the crater row of Þrengslaborgir and Lúdentarborgir, southeast of here, erupted and sent lava north across wet, marshy ground and a shallow body of water. Trapped water flashed to steam and forced its way up through the molten rock in vertical jets. Around each jet the lava chilled and hardened into a tube. When the ponded lava eventually drained away westward toward the lake basin, the fragile roof collapsed but the steam-hardened tubes stayed standing, leaving a forest of chimneys, buttresses and arches on a floor of rubble. Comparable structures are rare on land, and geologists more often find this kind of rootless pillar field on the ocean floor. The field has needed defending as well as explaining: drifting black sand off the lake shore was burying the formations by the middle of the twentieth century, and soil conservation work built barriers and planted the dunes to hold them, which is why the approach today crosses stabilised ground rather than open sand.

The stories attached to it. Icelandic folklore treats Dimmuborgir as a doorway rather than a rock formation. One tradition places the catacombs of hell here, the spot where Satan landed after being cast out of heaven. Better loved is the version that makes it home to Grýla, the ogress who eats badly behaved children, her idle husband Leppalúði, and their thirteen sons, the Jólasveinar or Yule Lads, who come down to the settlements one by one over the thirteen nights before Christmas with names like Door-Slammer, Sausage-Swiper and Window-Peeper. The connection is kept alive rather than merely recorded: in December the lads appear in costume among the pillars for local children, and the site runs a small seasonal programme.

On screen. Dimmuborgir provided the wildling camp in the third season of Game of Thrones, filmed during the Icelandic block in late 2012. This is where Mance Rayder's army of free folk is first seen properly, tents and cook-fires strung out among the lava towers, and where Jon Snow is brought before the King-Beyond-the-Wall. The pillars did the work that no set dressing could fake, giving the camp a sense of being wedged into hostile ground rather than pitched on an open field. Snow cover in the finished episodes was heavier than the site typically holds, and the giants and the scale of the horde are digital, but the towers, the arches and the black rubble underfoot are exactly as they stand.

What you actually see today. A free car park with a cafe and toilets sits at the entrance, and colour-coded trails leave from a single point. The small loop runs about 850 metres on relatively even gravel and covers the most photographed formations. The Church Circle, Kirkjuhringur, is roughly 2.3 kilometres and takes forty-five minutes to an hour; it leads to Kirkjan, a tall hollow arch whose ribbed interior really does read as a nave, and which is the single most recognisable feature in the field. Longer routes such as Krókastígur push deeper into the rubble and involve steps, loose footing and narrow passages. Ravens nest in the towers and are audible long before they are visible. Staying on the marked trails matters here: the moss and birch regenerate slowly and the ground is easily scarred.

Getting there and timing. Dimmuborgir is a five-minute drive from Grjótagjá and ten from the Mývatn Nature Baths, so most visitors take all three in one loop. Coaches on the Diamond Circle route arrive through the middle of the day, and the car park thins noticeably after five in the afternoon. In midsummer the midges that give Mývatn its name can be dense in still weather, and a head net costs little in Reykjahlíð. Winter turns the trails into packed snow and ice, and while the short loop is usually walkable with spikes, the longer circuits are best left to good conditions. Nothing here is charged for except the coffee.