What was it that motivated the people of the ancient city of Jiaohe to bury the bodies of 200 children in a mass grave about 700 years ago? Was it because of a plague affecting only children? Or was it, as the archaeologists who recently found the grave suspect, the outcome of a grisly religious ceremony aimed at bringing the city good luck?
If it was a desperate attempt to save the city, the magic failed because Jiaohe was caught up in the succession wars that followed the death of the mighty Genghis Khan in 1227, destroyed, and never lived in again.
The mysterious ruins of this place, founded about 2000 years ago, lie on top of a small plateau rising 30m out of the desert basin around the Chinese city of Turpan. These days its streets are wandered only by tourists.
The 380,000sq m site is dotted with the remains of the houses in which 5000 people once lived; doors, windows, walls and even fireplaces are still there to be seen.
I strolled up the main street past the thick walls of the roofless royal palace; behind me was the massive base of what once was the city watchtower; ahead was a massive building that once served as the main Buddhist monastery; beyond, the remains of a massive stupa still stood proudly.
The reason you can still visit this ancient city today is that it was built almost entirely out of earth in a place that sees only 16mm of rain a year.
At first, it seems, they created living spaces by digging down into the earth of the plateau. We were able to walk down the steps to see some of these multi-roomed cellar-like dwellings. But later, as the people of Jiaohe grew more ambitious, they also built upwards, using rammed earth walls and more conventional roofs.
After the place was sacked there were no bricks for locals to scavenge to build their houses and the lack of rain meant even after the passing of so many centuries, the remains of the city are still there to explore.
There has, of course, been some deterioration - you don't exactly expect the locals to suddenly appear from their homes and carry on living - but it's still a slightly eerie experience to stroll the streets of a city where everything stopped suddenly so many centuries ago.
Just 50km away across the Turpan basin lie the ruins of a much bigger city, Gaochang, which was destroyed at the same time in the same conflict.
Gaochang was built on the plains - in fact although it is many hundreds of kilometres from the sea it sits exactly at sea level - so it was protected by massive walls that still mostly loom around its edges today.
It is such a large site, 4 million square metres, that local people offer a donkey cart service to the most famous building, the city's great monastery, for just a couple of dollars.
I chose to walk so I could see the rest of this great sprawling ruin, but after a few minutes of wandering down dusty tracks in the blazing heat -- probably around 37C according to our guide -- I began to wish I'd taken the cart.
Gaochang was built entirely of adobe and, again, the earthen structures have withstood the passing of the centuries amazingly well.
Around a corner I came across the remains of another building, this one with three walls standing, with rectangular gaps where the doors were, and holes, which presumably once took the rafters that supported the roof.
However, when I finally got there, the monastery was undoubtedly the most spectacular sight. The main building was still enormous in spite of the loss of its roof and the partial erosion of the walls. Most remarkable was the domed prayer hall, which looked more like a Muslim mosque than a Buddhist structure.
Then again Bin, our guide, did explain that before its ruin Gaochang "was a very important stopping place on the Silk Road" so it would have been subject to an incredible array of cultural influences from Persia and Rome in the west to Mongolia and China in the east.
This particular monastery, he added, was "famous everywhere on the Silk Road for the very wise monks who taught here".
Perhaps the monks were indeed wise enough to take ideas from the other cultures passing along the great road and to adapt their building styles and other teachings accordingly.
They never met until 1200 years after they died but today they are a remarkably popular couple.
These two mummies, one male and one female, lie together in death as they never did in life at the Astana Tombs where for hundreds of years the people of the ruined city of Gaochang, and later those of the new city of Turpan that replaced it, were buried.
Excavation of the burial area has revealed hundreds of tombs - holding more than 1000 mummies - all built to much the same pattern.
Each was dug about 5m into the stony ground. A sloping entrance passage about 30m long leads to two small rooms, the first used to hold sacrifices, such as food or pottery, the second plastered and painted and used to hold the bodies.
The first we visited, which held the romantic couple, was lined with pictures of birds and flowers, including amazingly lifelike pheasants, geese and grouse.
"Do you see these?" said guide Bin excitedly. "Are they not beautiful? They look as fresh as when they were new."
And they did.
"And do you notice something else? These birds and plants are not from here. They are from the south. These are Han Chinese who moved here in the Tang Dynasty but wanted to think of their home when they died."
The two mummies were, Bin explained, merely examples put here for visitors to see.
Unlike Egyptian mummies, they were not intended to be preserved, "it just happened naturally because of the very dry climate, the wind which blows a lot and made them even drier and the very cold winters."
They were certainly remarkably well preserved by comparison with the mummies I've seen unwrapped in the Cairo Museum upon whom such care was lavished. The woman looked to be younger than the man, maybe 40 at the time of her death, according to Bin, with good teeth, fine features and her skin and hair still intact.
The man was older and taller and also remarkably well-preserved considering he had been lying in the desert for perhaps 1200 years.
There are three tombs are open to visitors at Astana; the others do not hold mummies, however one of particular interest was of a noted Chinese general, Zhang Xiona, who died in 633.
On its back wall were intriguing paintings illustrating Confucian precepts but when the tomb was opened by archaeologists that's all there was.
"The tomb was robbed," said Bin. "They took everything and the general's body too because he had gold teeth."
Fortunately the grave robbers did not take the stele that recorded details of who was buried there, which is how we know whose grave it was.
And, it seems, the mummy was later recovered - minus its gold teeth - because a day later we saw it on display at the wonderful Xinjiang Regional Museum in Urumqi.
The general was obviously a big, strong man and, despite the indignities his body had suffered, he still exuded power in death as he did in life.
Perhaps, I thought, that explains why the grave robbers left his mummy and fled: they were scared of what such a powerful spirit might do by way of revenge for taking his teeth.