The story of how it got there is pure outback improvisation. A quarry surveyor was handed the job of marking out the logo, and a grader driver named Allan Hoare then dragged it into the limestone over a single holiday weekend. The finished diamond measured 3.2 kilometres wide and 1.6 kilometres tall, the bold capitals of "READYMIX" running across the middle. Each letter stands 240 metres high and 180 metres wide, with lines twelve metres thick, wide enough to drive along. It was, at the time, the largest sign of its kind anywhere, and it earned a place in the Guinness Book of Records from 1972 to 1991, listed first as the world's largest advertising sign and later as the world's largest letters.

The location was no accident. Caiguna sat beneath a busy aerial highway, the radio beacon there serving as a navigational turning point for domestic flights crossing the continent. Roughly 400 passengers a day flew over this stretch of the Nullarbor, peering down at a plain that offered almost nothing to look at, until suddenly there was a company's name spelled out in the dirt below. It may be the most audacious billboard ever made: an advertisement aimed squarely at travellers who had nowhere else to direct their gaze, pitched to a captive audience seven kilometres up.

Like so much out here, the geoglyph was eventually undone by progress. As aircraft grew larger and navigation grew more precise, the flight paths to and from Perth shifted through the 1980s, swinging south to pass more directly over the Great Australian Bight. The airliners stopped flying over Caiguna, and the giant sign lost the very audience it had been built to reach. Without aircraft overhead and without maintenance, the lines began to fade back into the plain. For years the great logo was all but forgotten, a record-holder slowly dissolving in the scrub, until satellite mapping put the whole Earth on a screen and curious eyes rediscovered the diamond in the desert.

The Readymix logo predates the wave of monumental land art that Australia is now known for. The Mundi Man, a smiling stockman drawn across the Mundi Mundi Plains near Silverton, came in 2001, for the Year of the Outback. The Bunjil Geoglyph, a hundred-metre stone eagle honouring the Creator Spirit of the Wathaurong people, followed in 2006. The Nullarbor sign has another claim too, sometimes made and worth treating with care: it is occasionally called the first advertisement visible from space. In 2006 Kentucky Fried Chicken laid out a giant image of Colonel Sanders in the Nevada desert and billed it as the first brand visible from orbit, which, if the Readymix logo got there first, was simply not true. Records on the Nullarbor have a way of outlasting the people who set them.