HIDDEN BENEATH THE shifting sands of the Sahara is one of the great mysteries of archaeology. In 523 BC, according to legend, Persian King of Kings Cambyses II, son of Cyrus the Great the founder of second Iranian dynasty Achaemenids and the first Persian Empire, dispatched an army of 50,000 men to destroy the sacred oracle in Siwa that had been bad-mouthing him since his conquest of Egypt two years earlier. The soldiers of Imperial Army marched into the desert never to be seen again.

According to an account related by the 5th century BC Greek historian Herodotus, the army left Thebes (Luxor) and after seven days reached an inhabited oasis, probably Kharga. The 50,000 soldiers continued with their guides into the Great Sand Sea towards Siwa, but met their demise in a massive sandstorm.

"A great and violent south wind arose, which buried them in the masses of sand which it bore, and so they disappeared from sight," he recounts.

The fate of Emperor Cambyses' army is one of the great mysteries of archaeology. Attempts to find traces of it have ended in failure, and some historians suspect the tale was a fabrication, or at the very least a gross exaggeration.

Tourism companies, however, see it as a potential cash cow.

"It is a great opportunity," says Hisham Nessim, manager of Aqua Sun Resort. "We will give tourists a chance to participate in solving this ancient mystery and we will sell it as a touristic product."

Nessim, a former desert rally driver, leads the Egyptian Exploration Desert Team (EEDT), an exploratory "archaeological" mission funded entirely by private tourism firms. The plan, approved by the Ministry of Tourism, is to comb the Western Desert in 4WD vehicles packed with paying tourists hot on the trail of Cambyses' army.

Archaeologists have reacted with suspicion and horror.

"It's a very, very bad idea," contends Salima Ikram, associate professor of Egyptology at the American University in Cairo.

"The more people go trampling through the desert, the more they muck up the archaeological evidence." A foreign archaeologist, preferring not to be named, railed: "Do you think that if they find anything they will leave it intact? Of course not. They'll pick it up, manhandle it and take home a few souvenirs. This is just the sort of thing we don't need."

Nessim brushes off his critics, who he says are blowing things out of proportion.

Hundreds of desert safari expeditions take tourists to the Western Desert each year.

The only difference here is that the safari's route winds through areas deemed likely to contain remains of the lost army. Any evidence discovered will be referred to experts for analysis.

"My license is not to dig, so if I find something I must report it to the authorities," Nessim says, indicating that the Supreme Council of Antiquities (SCA) has no objections to the project. So far, Aqua Sun and co-sponsor Emeco Travel have organized two expeditions. Neither trip, each of which went ahead despite last-minute cancellations, made significant discoveries.

"We still have far to go. This secret has been hidden for over 2,000 years and we can't expect to find it in just two trips," says Nessim, who is reportedly cooperating with US space agency NASA to prepare a route for a third mission. "There are some places on the ground that I suspect and they [NASA] will check with satellites. "Remote sensors will also scan an area 50 km southwest of Siwa Oasis, where a Helwan University geological team prospecting for oil in 2000 discovered human bones, arrow-heads, frayed textiles and daggers in the dunes. The find sent shivers of excitement through the archaeological community, but an SCA team dispatched to excavate found nothing at the GPS coordinates they were given. The sand may have simply swallowed the evidence.

"The dunes in the Great Sand Sea move about 30-50 cm a year to the south and southeast because of the prevailing wind," explains geologist and EEDT guide Bahei El-Asawi. "It's hard to find anything, because the sand can cover one area and expose another."

The nature of the desert adds a challenge, says desert safari specialist Hani Zaki of Emeco Tours, who compares the search for Cambyses' army to finding a needle in a haystack.

"When you're in the Great Sand Sea you can look 360 and all you see is sand. There are no landmarks, mountains or anything," he says. "You don't see anybody and there is almost no sign of life, but there is plenty of natural beauty."

EEDT expeditions run between 10 and 22 days, traversing parts of the most beautiful and inhospitable desert in the world. The team's 4WD vehicles are specially equipped for deep desert exploration, carrying extra fuel, water, rations, parts and GPS equipment.

"There's always a risk, but it's greater if you're not following the rules, are not well-equipped or don't have experience," says Zaki. "Without risk it's not an adventure. Our role is to avoid major risks and minimize minor ones."

While Zaki is apologetic that the EEDT team does not include a professional archaeologist, he strongly rejects arguments that tourists are being deceived. Clients do not sign up for an archaeological dig, he says, they come to explore the desert with veteran guides "who know the desert like the back of their hand" and can provide valuable insights into desert history, geology, flora and fauna. The search for Cambyses' lost army is "just a theme," he says, insisting clients are aware that the chance of actually finding 50,000 desiccated soldiers and the bleached-white bones of their pack animals is remote. Instead, they hope to find traces like clay water vessels, trail markers, discarded weapons and - if lucky - the remains of a stray soldier.

Rival outfit Zarzora Expeditions is promoting similar themed trips, with itineraries that trace the steps of 19th Century German explorer Gerhard Rohlfs as well as Hungarian spy Count Laszlo Almasy (upon whom the 1996 film The English Patient was loosely based). The firm is also hoping to put together its own quest for Cambyses' lost Imperial army. "It is an irresistible marketing tool," says Wael Abed, the company's general manager.