The story begins almost unimaginably long ago. The Witwatersrand rocks were laid down across roughly 260 million years, starting nearly three billion years ago, when this part of Africa lay beneath a shallow sea geologists call the Witwatersrand Sea. The Kaapvaal craton beneath it is among the oldest stable pieces of crust anywhere on Earth. As the sea retreated and rivers braided across a flat coastal plain, they carried gold washed down from greenstone belts to the north and west, dropping it grain by grain into ancient deltas. Compressed and buried for eons, those gold-flecked gravels hardened into a conglomerate that miners call banket, after a sticky Afrikaner sweet it resembles. The gold had been waiting there, sealed in stone, since long before the first complex life appeared.

George Harrison's 1886 discovery on Langlaagte set off one of history's great gold rushes. Within a year a chaotic mining camp had a name, Johannesburg, and within a generation it was the largest city in southern Africa. The Main Reef, as the gold-bearing layer became known, dips underground and plunges for kilometers, and the mines chased it down. Some Witwatersrand shafts reach roughly four kilometers below the surface, where rock temperatures climb past 60 degrees Celsius and engineers pump ice underground just to keep the air breathable. More than 40,000 tonnes of gold have come out of these rocks since 1886, somewhere near 40 percent of all the gold ever mined on Earth. No other gold field comes close.

Few landforms have left so deep a mark on a nation's identity. The gold of the Rand underwrote South Africa's economy for more than a century, and when the republic was declared in 1961, the new currency took the ridge's name: the rand. The Witwatersrand gave its name to a great university, to whole regions, the East Rand and West Rand, and to the conurbation that sprawled along its length. It was once the W in PWV, the old shorthand for the Pretoria-Witwatersrand-Vereeniging heartland that became Gauteng province. A geological accident three billion years old became the engine of a modern state.

The wealth came at a cost the land is still paying. Pale yellow mine dumps, mountains of crushed and processed rock, ring southern Johannesburg like a strange artificial range. When the ore was hauled up from oxygen-starved depths, its iron pyrite began to react with air and rain, producing sulfuric acid. This acid mine drainage now seeps into groundwater, dissolving heavy metals, uranium among them, that the rock had safely locked away for billions of years. Below the city the hollowed-out ground occasionally shifts, opening sinkholes or trembling with small tremors. The ridge gave South Africa extraordinary fortune, and it left behind an equally extraordinary reckoning.