The borehole went down a thousand metres, cutting through six distinct geological layers, and the drilling took seventy-nine days. Kassel had to listen to every one of them. Into the finished shaft went 167 brass rods, screwed together end to end to form a single continuous rod five centimetres in diameter, one kilometre long and weighing eighteen tons. The top face was set flush with the surface of the ground and framed in a plate of red sandstone two metres square. Technical direction fell to the engineering firm of Dr Hans Jürgen Pickel of Kassel; the work was realised through documenta 6 under its director Manfred Schneckenburger, with the support of the Dia Art Foundation. It is a piece of civil engineering that produced no building, no shaft head, no visible infrastructure of any kind - only a sealed line of metal and the noise of putting it there.

The people of Kassel were not charmed. The bill came to 750,000 Deutsche Marks, something on the order of two million US dollars in 2016 values, and what the city got for it could not be seen. Weeks of drilling noise, and then a small brass circle in the pavement. The objection is entirely reasonable and entirely beside the point, because the invisibility is the work. What the money bought was not an object to look at but a fact to carry around: that directly beneath a particular patch of ground, a solid line of metal runs a kilometre down into the crust of the planet. De Maria did not build something to be admired. He altered what is underneath a place where people walk their dogs.

Documenta describes the piece as sitting inside all three of the significant artistic strategies of its period at once. It has the stripped geometry of Minimal art, the primacy of idea over object that defines Conceptual art, and the literal engagement with the earth that made Land art. Most works of the era settle for one. Ken Jennings, writing for Condé Nast Traveler, pointed out that the visible result closely resembles a kilometre-zero marker - the small metal plate European cities set into the ground to mark the point from which road distances are measured. That likeness is a good joke and probably not an accident. It looks exactly like a marker for a distance, and the distance it marks runs the wrong way.

Two years later, in 1979, De Maria built the counterpart in a street-level storefront in SoHo, New York. The Broken Kilometer uses the same brass, the same diameter and the same total length, but cut into 500 rods of two metres each and laid out on the floor in five rows of a hundred, under stadium lights, with the gaps between rods widening by five millimetres as the eye travels toward the back of the room. In Kassel the kilometre is completely hidden and completely intact. In New York it is completely visible and completely broken. The same quantity of metal, the same measurement, two opposite answers to the question of what you are allowed to see.

The Dia Art Foundation maintains The Vertical Earth Kilometer as one of the twelve locations and sites in its care, and it has been on permanent view - a phrase that asks for some generosity - since 1977. No visitor can verify any of it. You cannot see the rod, measure it, or watch it descend. What you can do is stand on Friedrichsplatz in front of the Fridericianum, where documenta has opened every five years since 1955, look down at a warm circle of metal in a square of red stone, and decide whether to believe that it continues for a thousand metres. Nearly everybody does. That decision, made quickly and without evidence by every person who walks across it, is the part of the sculpture that is actually above ground.