But, there is a group of people who do enjoy spending time on the strips of land that get exposed either side of the river by low tide – the mudlarks. All of them are attracted onto the mud by the same thing: the lure of the Big Find. It's a little-known fact, but the 95-mile foreshore of the tidal Thames is one of the richest archaeological sites in the country!

Anyone who wants to search the foreshore should have a permit from the Port of London Authority, the body that owns the majority of the muddy land that cradles the river. And although a Standard Permit allows the holder to take their search up to a depth of 7.5cm, it is only a Mudlark's Permit that allows any serious digging – and this premium permit is only available to members of the Society of Mudlarks. Although the Society has got a reputation for being rather secretive, this is probably just the result of people confusing mud with murkiness. Founded in 1980, the group is reportedly made up of around 70, mainly male, enthusiasts who get together to use their metal detectors on informal archaeological digs.

The terms of the mudlarking permits state that anything found in the mud that appears to be of historical interest must be taken to the Museum of London for identification and recording. Although the law in England states that any found object is the property of the landowner, in practice the Port of London Authority often lets mudlarks keep and profit from their finds. But, unless what's found is either extremely rare or an item of "treasure" (which is complex to define, but essentially a "hoard" of old coins, or a very old object partly made from precious metal), there isn't any money in mudlarking.

Hidden treasure

Apart from the fun of the dig, the only significant reward is being able to make a contribution to the bank of historical knowledge. Only a limited number of the 1,000 mudlark finds that the Museum of London assesses each year make it into their collection. In the last few years a medieval purse-frame, an iron dagger and a number of pilgrim badges (metal souvenirs bought to commemorate pilgrimages) have all been pulled out of the mud.

It's partly the nature of Thames mud that makes the foreshore so archeologically rich: its low oxygen content helps slow the rate of organic decay. On top of convenient chemistry, the river offers the added benefit to archaeologists that it was a main transport thoroughfare in medieval times, meaning lots of things got dropped into the water from ferries and cargo boats. More finds are guaranteed by the fact that domestic rubbish was used to backfill the timber embankments that lined the river up to about 1500 – as they were replaced, so wave upon wave of objects found their way down into the mud.

The word mudlark was originally used to name the children in Victorian London who were forced by desperate poverty to scour the Thames foreshore for items to sell for a few pence: bits of coal and rope, discarded iron, copper nails, old bones (ground up for fertiliser) and lost tools. We know about their lives from London Labour And The London Poor (1851), a book written by journalist and reformer Henry Mayhew.

• Day permits for mudlarking cost £7.50 and can be obtained by ringing the Port of London Authority (based in Gravesend) on 01474 562200. Another way to spend time on the foreshore is to volunteer for one of the clean-up programmes run by the environmental charity Thames 21 (www.thames21.org.uk).