Troldkirken is a langdysse — a long dolmen, not a passage grave — and it is one of the better-preserved examples in Denmark. The barrow runs about fifty metres, its edges marked out by forty-seven kerbstones set upright in a long rectangle. Sitting on top is a hexagonal chamber of massive uprights carrying a single broad, flat capstone. That capstone is the thing that stops visitors. There is no plausible way to look at it and not immediately wonder how it got there, which is precisely the question the troll story was invented to answer. Other dolmens and burial mounds stand scattered through the surrounding fields; this was a landscape people had been marking for a very long time.

The builders were the Funnelbeaker culture, the first farmers of southern Scandinavia, and they raised Troldkirken around 3500 BC. That date deserves a moment. It is roughly a thousand years before the Great Pyramid at Giza, and several centuries before the sarsen circle at Stonehenge. These were people who had only recently stopped hunting for a living, working with stone tools, timber levers and rope, moving multi-tonne blocks across boggy north Jutland because the dead required it. Whatever the chamber originally held is long gone. What survives is the effort — a communal statement of permanence built by a society that had been settled for only a few generations.

The Danish name works two ways. Trold means troll, but it also carries the sense of magic, sorcery and witchcraft, so Troldkirken reads simultaneously as the Church of the Troll and the Church of Sorcery — and the folklore obliges on both counts. Alongside the stone-throwing legend there is a second, quieter tale: a man who claimed to have watched the subterraneans holding a service of their own inside the dolmen. Both stories do the same work. They take a structure whose builders had been forgotten for millennia and file it under the supernatural, and they do it by reaching for the nearest available frame — a church, because a church is what a big stone building for solemn purposes is supposed to be.

Denmark began protecting its prehistory unusually early, and Troldkirken was there at the start. The site was placed under protection in 1809, making it one of the first prehistoric monuments in the country to be safeguarded by law, though the protection was not formally registered until 1850. That timing matters. Across most of Europe the nineteenth century was when megalithic monuments were being broken up for road metal and field walls; Denmark, in the middle of a national romantic revival, decided instead that its dolmens were part of the country's identity. Troldkirken is maintained today by the Historical Museum of Northern Jutland in Aalborg.

There is no visitor centre, no gate and no ticket. The dolmen sits in open country in Himmerland, roughly six kilometres east of Nibe and fifteen west of Aalborg, and you walk up to it the way people have for five and a half thousand years. On a grey Jutland afternoon, with wind moving through the grass and the kerbstones dark with rain, the troll explanation stops seeming quaint and starts seeming almost reasonable. The stones are too big, the arrangement too deliberate, the silence too complete. Somebody did this on purpose, and left no explanation at all.