By Alex Kirby
BBC News, Sanaa
Some blame khat for stifling Yemeni development
Sitting high up in the rocky mountains of northern Yemen, the country's capital Sanaa is finding that its dwindling water supply may not be able to sustain the ancient settlement.
I had almost walked right past the low doorway in the souk before I realised.
Then I stopped, peered into the gloom, and spotted it - a camel, sporting leather blinkers the size of small searchlights, circling the chamber, a wooden arm linking it to a central stone pillar which rotated as the animal tramped stolidly on.
It was a simple mill for producing sesame oil - and it would hardly have looked out of place at the time of Christ.
If you want local colour, then go to Sanaa. The entire old city is a Unesco World Heritage Site, complete with tall stuccoed houses whose perforated walls use the winds for natural ventilation.
There is the small square which covers the site of the 6th-Century Christian cathedral, though nothing now remains of its ebony and ivory pulpit or its crosses of silver and gold.
In the narrow lanes of the market you are pressed to buy a bag of dull golden crystals and your senses succumb to the smell of burning incense. To the Romans, Yemen was, after all, Arabia Felix, the home of frankincense and myrrh. They still make novel souvenirs.
Venture further, beyond the Old City's 30-foot-high (9-metre-high) clay walls, and there are plenty more reminders that Sanaa is one of a kind. There is the Military Museum, an eclectic assembly of old weapons (including a camel-mounted cannon), old motor cars, and relics of the British colony in Aden.
A chillingly topical touch is an illustration from the 1950s of an execution, the bloodied scimitar still in the swordsman's hand as the convict expires. Perhaps that explains why the museum is run by part of the interior ministry - the moral guidance department.
Sanaa is at least 2,500 years old. It claims to be the world's oldest inhabited city, though Damascus disputes that. But it is living on borrowed time.
For once climate change is not the presumed culprit. It is not natural causes at all - it is human behaviour.
However you do the sums, they just do not add up. There are nearly 20 million Yemenis - and the population doubles every 17 years.
Sanaa is a world heritage site
The country imports most of its food, largely because it has too little water to feed itself. Yemenis have about one-fiftieth as much water per head as the world average.
And, to confound confusion, insupportably large amounts of water go on a non-essential crop - khat.
Khat in today's Yemen is what smoking was in Britain a generation ago. Everywhere you go you find men with cheeks bulging bizarrely as they get their fix. It is a shrub whose leaves, when you chew them, can induce mild euphoria, excitement, hallucinations and even constipation.
It is increasingly popular in Yemen. While a few years ago men would spend a couple of hours a day on their habit, many now chew happily away for seven or eight hours.
Within the last five years or so khat use has become much more accepted among women. One young professional woman told me she chewed it perhaps once every three months, as a way of socialising.
More often, she said, would be too much. She blames the drug for what she says is Yemenis' failure to better themselves.
Moralising apart, khat is having a baleful effect on Yemen. Of the country's scarce water, 40% goes on irrigating khat - and khat cultivation is increasing by 10% to 15% a year.
You cannot blame the farmers. As one says, growing khat earns him 20 times as much as growing potatoes.
You probably should not blame the chewers either. In a country where almost half the people live on less than US$2 (£1) a day, you find your fun where you can.
The minister for water and the environment, Dr Abdulrahman al-Eryani, is an agricultural engineer - and he is a worried man.
There are concerns about khat's effect on Yemeni society
"The Sanaa basin is using water 10 times faster than Nature is replenishing it," he told me.
"And before long there won't even be enough to drink. I am not an optimist. I think many of the city's people will simply have to move away.
"The solution I am proposing is a very clear policy - a voluntary one - of reallocating people from here down to the Red Sea coast. We could use renewable energy there to desalinate sea water. And it would be cheaper than trying to provide enough water to Sanaa.
"This is not the first time that Yemenis have had to move to avoid disaster. It's happened many times in the last few thousand years, when Nature allowed the population to increase rapidly. This time, though, there are political frontiers in the way of an exodus."
The minister, himself a chewer, thinks weaning Yemenis off khat will be like breaking the tobacco habit.
"In time it won't be cool to chew," he says. "But time is what we don't have."
One of the ancient Arabic names of Sanaa translates as The Protected City. It looks as though the protection is running out.
From Our Own Correspondent was broadcast on Saturday, 7 April, 2007 at 1130 BST on BBC Radio 4. Please check the programme schedules for World Service transmission times.