There is a deep, unhealed historical wound in the UK's relations with China - a wound that most British people know nothing about, but which causes China great pain. It stems from the destruction in 1860 of the country's most beautiful palace.
It's been described as China's ground zero - a place that tells a story of cultural destruction that everyone in China knows about, but hardly anyone outside.
The palace's fate is bitterly resented in Chinese minds and constantly resurfaces in Chinese popular films, angry social media debates, and furious rows about international art sales.
And it has left a controversial legacy in British art collections - royal, military, private - full of looted objects.
By coincidence, one of the story's central characters is Lord Elgin - son of the man who removed the so-called "Elgin marbles" from Greece.
But there's a twist - a hidden side to this story - which I've been exploring as it involved my ancestor, Thomas Bowlby, one of the first British foreign correspondents.
His torture and death at Chinese hands - and the revenge taken by Britain, destroying the old Summer Palace in Beijing in 1860 - was a moment, says one scholar, that "changed world history".
These days the site is just ruins - piles of scorched masonry, lakes with overgrown plants, lawns with a few stones scattered where many buildings once stood. The site swarms with Chinese visitors, taken there as part of a government-sponsored "patriotic education" programme.
As everyone in China is taught, it was once the most beautiful collection of architecture and art in the country. Its Chinese name was Yuanmingyuan - Garden of Perfect Brightness - where Chinese emperors had built a huge complex of palaces and other fine buildings, and filled them with cultural treasures.
A new digital reconstruction by a team at Tsinghua University gives a vivid idea of what this extraordinary place looked like when, 155 years ago, a joint British-French army approached Beijing.
The army was sent towards the end of the Opium Wars to force Chinese imperial rulers to open up their country further to Western trade and influence. In command on the British side was the 8th Earl of Elgin, from one of the most famous families in British imperial history.
With him was Thomas William Bowlby of The Times. Elgin described Bowlby as "remarkably agreeable" and saw him as good for his image back in Britain, "the means of diffusing sound information on many points". The two men bonded on their journey towards China as cultural tourists, visiting the pyramids in Egypt.
Once they had arrived in China Bowlby wrote in his newspaper reports and private diary of his admiration for aspects of Chinese life - its fine buildings and "admirably cultivated gardens".
But cultural admiration was mixed with the harsh reality of a brutal war. He also reported a very one-sided military campaign as the Anglo-French force relentlessly approached Beijing. The British army's new Armstrong gun, he noted, inflicted "perfectly awful wounds" on the Chinese. "It smashes whatever it comes in contact with."
Because of this military power, Bowlby was confident that imperial China's rulers - "effete and faithless Mandarins", he called them - would "soon be suing for mercy". Eager to witness the war's end, he set off with a delegation of British and French officials - as well as escorting Indian army troops - to negotiate what they assumed would be the Chinese surrender.
It was to prove a fatal miscalculation.
Meanwhile, French troops reached Beijing and the Summer Palace, where they began helping themselves to porcelain, silks and ancient books - or simply destroying what they found.
British troops joined in when they arrived shortly afterwards. "Officers and men seemed to have been seized with temporary insanity," said one witness. "In body and soul they were absorbed in one pursuit which was plunder, plunder." When Lord Elgin arrived, he initially recorded his horror in his diary. "War is a hateful business. The more one sees of it, the more one detests it."
But loot was an established part of army pay, and Elgin helped organise an auction of the many thousands of works of art and other objects that had been taken. The army tradition was to share out the spoils, with officers and other ranks taking their cut, and some of the cash used to compensate the families of dead or wounded soldiers.
That might have been the end of the pillaging and destruction. But then news emerged that the delegation that had gone to negotiate Chinese surrender had been taken prisoner. Some members, including the journalist Bowlby, were tortured and murdered.
"For three days the men were tied up, and for three days their bandages were soaked with water so that they would become tighter and tighter," says historian Vera Schwarcz. "Every time they begged for water their mouths would be filled with dirt." Eventually several prisoners died, their corpses hardly recognisable.
In response, Lord Elgin ordered the British troops to burn down the entire Summer Palace complex.
The destruction, he wrote later, was intended "to mark, by a solemn act of retribution, the horror and indignation... with which we were inspired by the perpetration of a great crime".
He was worried about his reputation back in Britain, too. "What would the Times (newspaper) say of me," he reportedly told a French commander, "if I did not avenge its correspondent?"
Burning all the magnificent buildings took several days.
"Whenever I think of beauty and taste, of skill and antiquity while I live, wrote James M'Ghee, chaplain to the British forces, "I shall see before my mind's eye some scene from those grounds, those palaces, and ever regret the stern but just necessity which laid them in ashes."
I visited the current Lord Elgin, at his ancestral home in Scotland, to ask how he explained what had happened in 1860. He showed me, from family archives, a picture sketched by a British officer of the return of Bowlby's mangled body in a coffin to British headquarters.
"There are things that perhaps you might have done differently," he says of his ancestor. "At the same time you've got to judge what was the feeling - intense feeling - at that particular moment."
China rejects such explanations.
"This is what they say to justify their actions," says Wang Daocheng, a leading Chinese scholar of these events. "That's the way they try to maintain the so-called moral high ground."
Soon after the Summer Palace's destruction in 1860, the 8th Earl of Elgin made a triumphant entry to the centre of Beijing, his procession symbolising British and Western domination - and Chinese humiliation.
For some time afterwards, memory of what happened faded in Chinese minds as the country went through modernisation, the end of imperial rule, war and communist takeover. Indeed in the communist-led Cultural Revolution of the 1960s, says historian Vera Schwarcz, "some remnants of the Summer Palace were literally slashed with knives by Red Guards". They hated reminders of the imperial past.
Since the 1989 Tiananmen Square protests against communist rule, however, China's leadership has tried to reinforce its authority by encouraging patriotic pride in the country's history - and teaching citizens that only strong government today can prevent a repeat of the 19th Century humiliation by outsiders. The ruined site of the old Summer Palace offers an ideal place to make this point.
China is also focusing increasingly on all the art that was looted by French and British forces - and taken to Europe. It was widely traded and still sits in all kinds of private and public collections.
A recent film by the martial arts star Jackie Chan has stirred up a sense of resentment in millions of Chinese minds. He plays a daredevil hero attempting to recover from evil Western collectors and museums a set of bronze carved zodiac animal heads - among the most famous items looted from the Summer Palace.
The real Chinese loot investigators appear no less determined, even if they don't resort to violence.
"We're making a plan to start a series of actions to recover these antiques and get them back to China," says Niu Xianfeng, general director of the National Treasures Fund, affiliated to the Chinese Ministry of Culture.
"China will never give up the right to bring these looted or stolen treasures back."
Liu Yang, a researcher who has spent 15 years tracking down the artworks, says "British museums never reply" when he writes to ask what they have. But he has collected hundreds of images of looted items on his computer.
He even has pictures of a Pekinese dog, taken by a British soldier from Yuanmingyuan, and given to Queen Victoria. It was the first of its breed to come to Britain - and was named "Looty".
A portrait of Looty is still in the Royal art collection, though later newspaper reports said the dog was ostracised by other royal dogs because of its "Oriental habits and appearance", and had to be moved from Buckingham Palace to Sandringham.
The Royal collection has several other items thought to be connected with the Beijing Summer Palace, including Chinese imperial sceptres, brass plaques and a mahogany screen.
The Wallace Collection in London has magnificent imperial vases from the palace.
British military museums have many items too. At the Royal Engineers' museum in Kent deputy curator James Scott showed me a beautiful jade ornament brought back from the 1860 campaign. There are also parts of a Chinese imperial throne acquired by the officer Charles Gordon (later famous for his death in Khartoum) - used for many decades as part of the furniture in the officers' mess.
Labelling these items is a sensitive matter. "We don't actually mention the word loot at all. We try to keep the interpretation as neutral as possible," says Scott.
Similar sensitivities are needed by auctioneers, who can make huge profits when items originally taken from the Summer Palace are re-sold today. Proof of their origin as part of the Chinese imperial collection - such as inscriptions by made by the soldiers who looted them - hugely increases their potential value.
Some newly wealthy Chinese have bid for such items. But having to pay for art that was stolen - as many Chinese see it - causes increasing resentment.
And what of the Elgin family? Does today's Lord Elgin think art should be returned to China?
"It's a very good arguing point" he concedes. But "the beauty of something is inherent in it wherever it happens to be".
At the family home, he showed me a magnificent pair of stork sculptures in bronze, originally given by the Japanese emperor to his Chinese counterpart, and then brought back by the 8th Earl of Elgin from Beijing after his China campaign.
"These things happen," he says of the 1860 events. "It's important to go ahead, rather than look back all the time."
And the British-Chinese relationship is certainly looking to a new future. It was symbolised for Lord Elgin when Chinese workmen arrived at Rosyth naval dockyard, very near his home, to install a huge crane, needed to build Britain's new aircraft carriers.
The workmen started helping themselves to Lord Elgin's oilseed rape crop - a Chinese delicacy. He objected at first, but then came to see it as a kind of "tit for tat".
But the Chinese want a lot more compensation than that for what happened in 1860.
They realise it will be very difficult, so long after the event, to retrieve what was taken from the Summer Palace, but they are keen, at least, for more open acknowledgement by Britain of what was done.
The French, who joined in the looting of the palace, have been more open about their regret. "We call ourselves civilised and them barbarians," wrote the outraged author, Victor Hugo, about the destruction of the Summer Palace. "Here is what Civilisation has done to Barbarity."
A recent French book, The Sack of the Summer Palace by Bernard Brizay, has been translated into Chinese and warmly received in China.
In the UK, the emphasis is overwhelmingly on the future - on Chinese investment and trade - not the past.
Prime Minister David Cameron has spoken of a new "partnership of mutual respect and understanding" with China and it's against this backdrop that the Duke of Cambridge will soon visit the country. But developing this relationship may one day mean engaging with the painful past that China has not forgotten.
Even in China, though, memory is selective.
Some of the 1860 history has already been buried, as I discovered when I went to look for Thomas Bowlby's grave. Instead of a cemetery, all I found was a golf driving range.
Listen to Chris Bowlby's Radio 3 documentary, Palace of Shame, on the BBC iPlayer
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