Dimmuborgir

Dark cities, or dark castles, a collapsed lava tube east of Mývatn, left by a lava lake that flowed in from the Þrengslaborgir and Lúdentsborgir crater row about 2,300 years ago. The lava pooled over marshy ground; the water beneath boiled, and the vapour rising through it formed hollow pillars from drainpipe width to several metres across. When the lava drained away towards lower ground the top crust fell in, but the pillars stayed standing. The lake must have been at least ten metres deep, judging by the tallest of them. Several chambers are big enough to stand inside.