Most of the stuff earmarked for Waste Isolation Pilot Plant (WIPP) in the New Mexico desert is plutonium-contaminated detritus which emits relatively low quantities of radioactivity gloves, bits of drill, flasks, valves, rags, test-tubes, pipes, sludge, shoes, lab coats, and so on. But some of it is the most threatening material on earth. For this network of tunnelled-out salt corridors, 26 miles east of Carlsbad, is to become, if the US government has its way, home for all the radioactive garbage created by US weapons plants during the Cold War. This includes 24,000 soft steel 55-gallon caskets containing waste that can kill someone within half an hour of exposure. The material will be radioactive for at least 10,000 years and, in some cases, far longer.

And therein lies the rub. Once the WIPP complex is filled, sometime in the next 50 years, the plant above ground will be vacated and returned to the desert. Its small clump of sand-coloured administration buildings, surrounded by barbed wire, private guards, and spotlights, will be removed.

How, then, should the rulers of today warn future generations of the filthy brew that they have buried beneath their feet? How will they stop them digging into it?

A response of sorts has come from the US Congress. As a condition for permitting the site to go ahead, it insisted that a warning sign should be erected when it closes down. This would have to be capable of alerting future generations of the risk of opening up this unwanted tomb. It would be the most momentous Keep Out sign in history, a statement so forceful that it would drive people or any other form of intelligent life away from the area until AD 12000.

Yet this ruling raised more questions that it solved. How do you create a sign that is comprehensible over a hundred centuries? Why should anyone assume that it is possible to create a structure that will outlive any previous empire from Mongolian and Ming to Roman and Russian? It is not as if the meaning of Stonehenge, which is a mere 3,500 years old, is crystal clear to modern man.

And how do you physically go about building something that can survive for so long, without being destroyed by sandstorms, or tumbled by an earthquake, let alone nuclear war? Of the original Seven Wonders of the World, only one still stands Khufus pyramid at Giza in Egypt.

Nor is it easy to see how you ensure that any sizeable monument, however forbidding, will not fill gold-diggers or future archaeologists with such curiosity that they start digging underneath it. Over the ages, few historic sites have been spared intrusion from bounty-hunters, vandals or prying scholars. Worse, the dump lies in an area of south-eastern New Mexico which is dotted with oil and hydrogen wells, and basalt mines. A prospector could happen upon its contents while boring for minerals.

To tackle these issues, the Sandia National Laboratory in Albuquerque, the nuclear weapons development and research agency which is overseeing the project for the US Department of Energy, convened a panel of 13 experts. They included anthropologists, materials scientists, astronomers, a psychologist, an architect and a linguist. Their deliberations were closely followed by nuclear regulatory agencies worldwide, including Britains. It was the first time anyone, anywhere, had explored the issue in such detail.

The result was a set of suggestions that would have brought a blush of pride to the cheeks of any Dr Who scriptwriter. The panel divided into two teams. One offered a number of alternatives. These included: a Landscape of Thorns a square mile of randomly-spaced 80ft basalt spikes which jut out of the ground at different angles; Menacing Earthworks giant earthen berms surrounding a 2,000ft map of the world displaying all the planets nuclear waste dumps; a Black Hole a huge slab of black concrete that absorbs so much solar heat that it is impossible to approach.

The second team favoured a field of 50ft granite obelisks (Washington Monument-style structures that would be large enough not to be buried by sand dunes). Beneath the earth, there would be further warning emblems in case someone made off with the surface monuments, just as they did in the past with Cleopatras Needles (built in Heliopolis by Thutmose III, now in London and New York) or the Elgin Marbles. There would also be a device to ensure that the site showed up on radar.

We toyed with the idea of actually burying some treasure 20 feet down so that anyone digging would think that they had found whatever they were looking for, said Professor Frank Drake, an astrophysicist from the University of California, but we dismissed that as too kooky. There was a lot of discussion about the morality of the whole thing. At one stage we thought that the site should not be marked at all, but we decided that was unethical.

The panellists put forward proposals for figurative warnings cartoons showing a stick figure collapsing from the effects of radiation, or a contorted face like Munchs The Scream. They also recommended written messages lengthy explanations of the dumps contents in English, French, Arabic, Russian, Chinese, Spanish and, possibly, the language of the local Mescalero Apache Indians.

These findings have formed the basis for a single report compiled by the Sandia laboratory, which has been passed to the Department of Energy. The final decision about what to erect will take some time. At the moment the WIPP project has been held up in the federal courts by a series of lawsuits over the safety of its operation, filed by environmental groups and the state of New Mexico.

But few doubt that the lorries (on average five a day) of nuclear-poisoned trash will eventually start rumbling across the southern desert, laden with waste destined for the ancient seabed. The US government is unlikely to abandon its newly-dug underworld. They drilled 2,150ft into the ground to build it.

The government has already spent dollars 1.5bn on the project. Many believe it will be a prototype for future nuclear waste disposal, an example that will be held up to justify the further use of nuclear power. Present estimates suggest an opening date of mid-1998.

The effectiveness of the so-called Sign of the Ages to warn the heirs to the planet away from their unwanted inheritance remains in doubt. It is hard to believe that, however expert the Americans claim to be, they have the slightest idea of how their utterances will be received in 200 years time, let alone in another epoch.

This week I was invited to go down into the depths of WIPP. It only took a few minutes to drop in a cage elevator from the bright afternoon to the hot, starkly-lit underworld. Wearing miners helmets and carrying oxygen masks, our group a party of foreign correspondents climbed into souped-up golf carts and drove through a subterranean maze of tunnels and vaults.

As we rattled along through several of the 13 miles of saline corridors, our guide, a government scientist called Wendell Wuert held forth on the capacity of salt to swallow up practically anything you decide to dump in it. Salt deposits drift slowly over the years, he explained, sealing up any cracks, holes, or even giant chambers that anyone might have made.

There was almost no chance, said Wuert, that anything nasty might seep into back into the biosphere. We are using the best science available, he assured us. Its just a question of convincing people.

The day after visiting WIPP, I flew back to Los Angeles. My taxi driver at the airport was from Lithuania, an erstwhile resident of the evil empire that helped to generate the US nuclear weapons. I asked him to go to Encino, the neighbourhood where I live. Cinema? he replied. I tried signs and gestures. Only after 15 minutes of garbled conversation did we straighten matters out. What chance would we have had, separated by 10,000 years?