Nicholas Schoon is beguiled by a bog that has a hold on its admirers, and their boots
THE little-known mud springs of Wootton Bassett in Wiltshire are a weird, beguiling natural phenomenon. They are not much to look at and The Independent found them rather dangerous. But they do steadily disgorge the most exquisite fossils along with vicious grey ooze and they seem to be unique, not just in Britain but in the world.

A visit was called for after we heard that Wootton Bassett's town council's finance and general purpose committee had put forward the springs for World Heritage status. The council hopes to persuade Cabinet culture supremo Chris Smith to seek Unesco's highest accolade for them, putting this little bog alongside Britain's 17 existing World Heritage Sites which include Canterbury Cathedral, Stonehenge and the Georgian glory of Bath.

We were shown round by councillor Eric Hodges, a 68- year-old retired biology teacher who is the spring's greatest enthusiast. A short tramp along a canal tow-path, a sharp right- hand turn towards the copse and there before our eyes was the largest of the springs - a big pond full of nothing but grey mud, surrounded by barbed wire fencing with a danger sign.

The councillor nimbly crossed the wire and guided us around, showing off several of the vegetation-surrounded vents from which Jurassic slime wells ceaselessly from below. He took me to Hancock's Water, a neighbouring stream into which the mud pours and where fossils can be found.

Back in 1974 workers from a sewage works a few hundred metres down stream noticed the flow of mud down the water course and went to investigate the source. They decided to dig down into one of the vents, but that caused a terrifying blow out. The earth shook and slime jetted high in the air plastering the surrounding trees.

Clearly, what had been thought of as merely a local bog was altogether more potent. Wiltshire County Council tried to plug one of the vents by dropping 100 tons of rubble down it. That disappeared without trace and the area of mud expanded, turning it into the slime pond you can see today.

Investigations by scientists from the British Geological Survey and elsewhere have begun to unravel how the springs work. Their starting point is the ground-water which, under pressure, moves upwards out of a layer of porous limestone, the Coral Rag, lying 70ft deep.

This rising water moves into a layer of clay deposited in a shallow sea about 150 million years ago. It appears that the water has opened large caverns in the solid clay, full of mud. This breaks out at the surface vents forming large blisters, where some of the mud congeals and some oozes into the stream. All this and more Mr Hodges explained as we explored the springs - and then disaster struck. He misjudged a step and plunged deep into mud which nearly covered the top of both Wellington boots. We managed to free first him then one of the boots, but the other proved quite impossible to wrestle out and lies there still.

As he hobbled away unbooted, he explained his vision for the prospective World Heritage site. There would be secure viewing platforms from which visitors could gaze at the mud, while large interpretation boards would explain the working and history of the springs.

The ooze is popular at Helen's Unisex Hair Salon in Wootton Bassett, where manageress Amanda Hamilton offers it free as a rejuvenating face mask. "After 15 minutes it dries and shrinks to form a really tight mask," she said. "It exfoliates and plumps up the skin a treat."

At the Natural Environment Research Council's headquarters in nearby Swindon, Neville Hollingworth, a senior geologist, showed me fossils he had found at the springs. The ammonites, little molluscs, still had their original mother of pearl shells with a lustre and hue which made them look like strange jewellery.

There were ancient Jurassic shrimps and even the vertebra of an ichthyosaur, a gigantic sea reptile. On one of his early fossil-hunting trips there, Mr Hollingworth fell into a vent up to his chest. "I was absolutely terrified," he said.

The Government designated the mud springs an Official Site of Special Scientific Interest earlier this year. But even though Chris Smith has said he wants to see a few more British World Heritage sites which are natural rather than man-made, it is hard to see Wootton Bassett's mud springs mounting a serious challenge to places such as Oxford, which are also trying to get on this most select of lists. World Heritage sites have to be of outstanding value significance to all mankind. "We'll consider any site ... but we have been stressing that the criteria are extremely strict," said a spokeswoman for the Department of Culture.

Undaunted, Mr Hodges insists: "Our mud springs are absolutely unusual and unique."