By Richard Hamilton
BBC News in Nouadhibou, Mauritania
Museums are reluctant to buy meteorites coming from North Africa
Mauritania's second city, Nouadhibou, is a strange frontier town that feels like the end of the world.
In the market you can buy anything from chickens and goats to traditional herbal remedies and magic potions.
But perhaps the weirdest objects of all on sale are meteorites, some of the rarest rocks on earth that land from space in the wastelands of the Sahara desert.
"People come here because they collect stones and rocks," one market trader told me.
"Nomads bring them from the desert. They say the stones have fallen from the sky."
He said that foreigners had become very interested in these stones recently, knowing they were valuable.
He said they told him they wanted to have them as decoration in their homes.
But he added that the nomadic people who found them in the desert did not know their true value.
Many are smuggled to collectors around the world and experts are worried that valuable scientific research is being lost as a result.
Meteorites land anywhere on earth and often end up in the oceans.
Nomads bring meteorites to lawless market towns like Nouadhibou
On land, they are most likely to be found near the Equator because it has the largest surface area, and particularly in the Sahara, as the dry climate helps to preserve them.
Providing they land on the flat hard pad rather than the soft sand dunes, meteorites are easy to spot in the desert and people will sometimes go in search of them if they see a meteor shower at night.
Caroline Smith, curator of the meteorite collection in London's Natural History Museum, says some of her meteorites are extremely valuable because, at 4,500 million years old, they are the oldest objects known to man.
"Meteorites are vitally important to scientists because we can understand our solar system and our own planet so much better by understanding and studying meteorites," she says.
"They can tell us a wealth of information, such as the very first stages of the formation of the solar system over four and a half billion years ago".
But Dr Smith is worried that the craze for meteorite collecting is having a damaging effect on scientific research.
"The commercial value of meteorites has now been realized," she says.
"It has affected our work because we are now competing against private collectors to obtain material for our research."
Institutions like the Natural History Museum will now not accept meteorites if they do not know where they came from, suspecting that they may have been obtained illegally.
Other organisations have taken similar measures: the Smithsonian Institute in the United States will not import meteorites from North Africa, knowing that the region has become a favourite target for dealers and smugglers.
Meteorites are well preserved in the dry desert conditions of the Sahara
John Thorne is a young American traveller who has had first-hand experience of smuggling meteorites from Mauritania.
"I actually helped out some friends who live in Algeria and who wanted to sell some of their meteorites abroad. They needed a hand getting them out of the country," he told me.
"We decided that we would meet in Nouadhibou because it's lawless: the only law there is money and you can do whatever you want, as long as you're prepared to pay the right person for it."
"In the end we didn't have to pay any bribes and it was easier than I expected. I thought I would be lugging kilos of meteorites out of the country by plane, but we found a shipping company at the port that was happy to ship them out without asking any questions."
The extraordinary voyages of meteorites, which started thousands of millions of years ago, now often come to their final destination at a very surreal place, the internet auction site eBay.
There are dozens of advertisements selling meteorites, often for no more than $10 or $20.
But scientists oppose this marketplace. They want to put a stop to the trade because they're losing valuable material to the collectors. They also say that local scientists and museums should have the privilege of looking after meteorites in their country of origin.