The Danish winter gave Poulsen the only crane he was ever going to get. When Klejtrup Sø froze hard enough to hold weight, he hauled stones out across the surface and positioned them exactly where a coast needed to be — some of them weighing more than two tonnes, moved with primitive tools and whatever leverage a farmer can improvise. Then he waited. As spring came and the ice thinned, each stone sank through it and tilted down onto the lake bed, landing more or less where he had left it. A frozen lake is a flat, frictionless working surface that quietly disposes of itself. He did not invent this trick so much as notice that his climate was offering it, one season at a time, for free.

Poulsen worked to a real scale, not an approximation. One degree of latitude — 111 kilometres on the ground — is 27 centimetres on his map. The finished world measures 45 by 90 metres and covers over 4,000 square metres, an acre of water rearranged into continents. Denmark, at that reduction, is a couple of hand-spans of grass. Africa is something you step across in a few strides. The continents sit as low green islands with lake between them, so a visitor crosses oceans on foot where the shallows allow and by boat where they do not. The conceit only works if the maker commits to it completely, and Poulsen did: no shortcuts, no schematic gestures, just latitude and longitude carried out in fieldstone.

Do the arithmetic and it becomes something other than a curiosity. Poulsen was born in 1888. He started in 1944 and he was still at it when he died in 1969 at the age of eighty-one — a quarter of a century, worked in the months a farm could spare, with no commission, no patron, no deadline and no client but himself. Nothing in the record explains why beyond the stone shaped like Jutland. Perhaps nothing more is needed. Some obsessions do not have reasons; they have dimensions, and the dimensions are the argument. Twenty-five winters of hauling rock onto ice is not a hobby. It is a life bent quietly around a single idea.

It survived him. The World Map is now the centrepiece of an attraction that draws roughly 40,000 visitors a year, most of them Danish, and the park itself reports well over fifty thousand these days. A family day out has grown up around the edges — a maze, mini-golf, boats, animals, somewhere to eat — and the site remains in private hands. None of that has changed the thing at the centre. You can still walk from Denmark to Australia in about a minute, past coastlines a farmer set one stone at a time, and feel the planet briefly shrink to a size a person can hold. That was the point on the ice in 1944, and it is still the point now.