I had always imagined the Terracotta Army in the tomb of China’s first emperor to be the expression of one man’s sublime madness, a posthumous game of toy soldiers on a megalomaniac scale. Who but a lunatic would force an army of workers to build an 8,000-strong army from baked mud to march him to the afterlife? The photographs of the inhuman warriors always seemed to me slightly chilling, row upon row of identical imperial army grunts two millennia old, entombed by a dictator’s pride.

That was before I met them. Last month I travelled to Xi’an, stood in the vast hangar that houses the excavation site, and looked down on an ancient host, not of impersonal clay models, but of people. What no photograph can prepare one for is the sheer humanity of the Terracotta Army, for these are not mass-produced dummies, but individuals.

Each is different. Artists long dead invested each with personality and purpose. There are the infantrymen, the officers, the archers, the cavalrymen, but also acrobats, musicians, wrestlers and civil servants, pompous with folded arms. Look carefully, and you will see China’s different ethnic types represented in the ranks; look closer still, and there are distinct flickers of character and expression: these men are angry, smiling, fierce, arrogant, even a little alarmed. But above all, they are patient. This is an army in attendance, waiting for eternity.

The Terracotta Army is the most important archaeological discovery of recent times, but also the greatest collective work of art ever produced, comprising thousands of subtly individual artworks. Here is humour amid the hubris; dictatorship but also a sort of creative equality. This is a necro-army drawn up to honour a mighty king, which stands today as a monument to countless unnamed artist-slaves.

Next week the British Museum will put on display 20 of the warriors, the largest detachment of the army ever to travel abroad, in what is expected to be the most popular exhibition mounted by the museum. Visitors will stream past the warriors at the rate of 400 an hour.

Qin Shihuangdi, China’s first emperor, would have been appalled.

His replica army was created for the pleasure of one man alone: himself. He did not intend it to be seen by human eyes, in this world. The exhibition raises important questions about what art is for: whether it is a luxury to be enjoyed by a powerful elite, or a pleasure that should shared as widely as possible. Qin undoubtedly believed in the former principle. Yet surely the artists who put smiles and frowns on the faces of the warriors, who fashioned bronze birds to amuse the emperor and an umbrella in case it rained in the hereafter did so as an elemental form of self-expression and in the expectation that their work would be seen and enjoyed by others.

China’s decision to send its ancient soldiers abroad is part of a new “terracotta diplomacy”, a desire to demonstrate openness to the world by making its treasures and culture more accessible. Yet the wonders on display in Xi’an and London represent only a tiny fraction of what China could, and should, uncover at the same site.

Like many an autocrat, Qin was obsessed with a quest for immortality. He set his alchemists to work brewing up the elixir of life, and sent explorers to discover the secret in foreign lands, on a promise that they would be executed if they failed; sensibly, few came back. And he created the most elaborate grave ever imagined, of which the Terracotta Army so far unearthed is merely the vanguard. Archaeologists estimate that the burial complex could extend over 30 square miles.

The emperor’s quest for eternal life proved fatal, when the mercury pills designed for this purpose apparently killed him. The historian Sima Qian, writing a century later, described how Qin was buried in a bronze casket, wearing jade and gold, placed in a perfect replica world with a pearl-studded ceiling symbolising the firmament and China’s rivers represented by streams of mercury. The chamber was sheathed in copper, he wrote, and guarded by booby-trap crossbows to deter grave robbers.

Using remote sensing technology and computerised imagery, scientists have identified a giant tomb beneath a man-made mound on the site. Tests have shown high levels of mercury in the soil. Qin’s great burial chamber lies, in all probability, beneath a few yards of earth, and there it will remain unless there is a change of heart in Beijing.

Chinese museum officials have said there are no plans to explore the tomb; the most important archaeological site in the world remains unexcavated, its secrets locked away. The officials cite concern over “desecrating” a grave and lack of technical equipment, but the unwillingness to probe deeper may reflect a more ingrained preference for hoarding over display. China hates to be hurried: it has taken more than 30 years to carry out the limited excavations so far completed.

Yet at a time when all eyes are turned towards Beijing for the Olympic Games, it would be hard to imagine a more dramatic statement of openness and cultural sophistication than an announcement that the great mausoleum of Qin will finally be excavated, preferably by a Chinese-led international team of archaeologists.

Such a gesture would enhance China’s international reputation more than any number of gold medals, parades or new buildings. Opening up Qin’s tomb would demonstrate that great art is for all; it would bring recognition to the Chinese craftsmen and artists who laboured to create such beauty; it would add immeasurably to our understanding of an extraordinary civilisation, and end the patient wait of the terracotta soldiers who have stood guard for so long. Finally, perhaps, it would bring Qin Shihuangdi, a great and terrible king who is still almost unknown in the West, the immortality he craved.

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