Somewhere beneath the baked plains of Sindh in southern Pakistan lies a city that had no business existing when it did. Around 2500 BCE, when most of humanity was still scratching a living from scattered villages, Mohenjo-daro was already home to roughly forty thousand people, laid out on a precise grid, supplied by hundreds of wells, and drained by covered sewers that would not look out of place in a Roman town built two thousand years later. It is one of the oldest major cities on earth, and almost nobody built anything to match it for millennia.

The civilisation that raised Mohenjo-daro is known today as the Indus Valley Civilisation, sometimes called the Harappan Civilisation, which emerged around 3000 BCE from earlier farming cultures along the Indus River. At its peak it stretched across most of what is now Pakistan and north-western India, reaching west toward the Iranian border and south into Gujarat. Its major centres included Harappa, Lothal, and Dholavira, among others. Mohenjo-daro was the grandest of them all.

The city sat on a natural ridge above the Indus flood plain, roughly twenty-eight kilometres from the modern town of Larkana. Builders chose the site deliberately, and they built quickly: the water supply system and its wells were among the very first structures raised. From the beginning, this was a city designed around the needs of a large population.

What the excavations eventually uncovered was a settlement of astonishing regularity. Buildings were arranged on a strict grid, most of them constructed from fired and mortared brick of a standard size, strong enough to survive four and a half thousand years underground. The city divided into two distinct zones: a raised Citadel mound, about twelve metres high, containing public buildings, and a sprawling Lower City where most people lived and worked.

The Citadel held a structure that has become one of the most famous things about the site: the Great Bath. Steps lead down from a colonnaded courtyard into a brick-lined pool about twelve metres long, sealed with a lining of bitumen to make it watertight. Scholars believe it was used for ritual purification, though no text survives to confirm it. Nearby stood a large building once confidently labelled a Great Granary, though later archaeologists pointed out that no actual grain was ever found inside it, and its true purpose remains open.

Ordinary houses had inner courtyards, doors that opened onto side lanes rather than main streets, and in the more prosperous cases, rooms set aside for bathing. One building contained what appears to have been an underground furnace for heated bathing. Over seven hundred wells have been counted across the site, one for roughly every three houses: a ratio unmatched by Egypt or Mesopotamia at the same period. The circular brick well design used here may well be an Indus invention, with no earlier examples found anywhere else.

The city also had a centralised drainage system running beneath its main streets, carrying waste toward the river. Large platforms may have served as flood defences. According to one theory, Mohenjo-daro was inundated and rebuilt perhaps six times, its citizens patiently raising the city again on the same footprint each time the waters receded.

The objects recovered from Mohenjo-daro are the closest thing we have to meeting its people. A small bronze statuette found in 1926, just ten and a half centimetres tall, shows a young woman standing with one hand on her hip, bangles stacked the length of her arm. Mortimer Wheeler, the British archaeologist who excavated the site in 1945, described her as about fifteen years old, utterly confident of herself and the world. She is known today simply as the Dancing Girl, and she now sits in the National Museum in New Delhi.

A year after the Dancing Girl's discovery, a seated soapstone figure was found in a building with unusually ornate brickwork. Bearded, wearing a cloak decorated with trefoil and circle motifs that still carry faint traces of red, he is known as the Priest-King, though there is no evidence that priests or kings ruled Mohenjo-daro at all. The name stuck anyway. Pakistan asked for and received this figure after independence; India kept the Dancing Girl. In some cases the division of artefacts between the two new nations was startlingly literal: beads from necklaces were counted out into two equal piles.

A seal found at the site shows a cross-legged figure surrounded by animals, interpreted by some scholars as a yogi and by others as a proto-Shiva, a forerunner of the Hindu god later known as Lord of Animals. Whether or not the connection holds, the image has fascinated scholars of South Asian religion ever since. The site also produced ivory rulers divided into precise decimal units, predating the modern metric system by three thousand years, and copper tablets inscribed with a script that has never been deciphered.

Around 1700 BCE, Mohenjo-daro was abandoned. The Indus Civilisation was collapsing across its entire range. The causes are still debated: repeated flooding, soil exhaustion from overcultivation, a drying climate, or some combination of all three. The city was not burned or sacked. It was simply left. The grid of streets, the wells, the Great Bath, and the Priest-King sat undisturbed beneath accumulating silt for roughly three thousand seven hundred years.

In 1919 and 1920, an officer of the Archaeological Survey of India named R. D. Banerji visited the site looking for a known Buddhist structure. He found a flint scraper that told him something far older lay beneath his feet. Large-scale excavations followed through the 1920s and 1930s, and again in 1945 under Wheeler. The last major digs ran in 1964 and 1965. After that, excavation was banned: the exposed structures were weathering too fast, and the only work permitted since has been salvage, survey, and conservation. A core drilling carried out in 2015 suggested the buried city is larger than anything yet uncovered.