The ground beneath your feet is not as it seems. Don't be fooled by the solid appearance. Below us is a thriving, man-made realm that is anything but inert and constant. Shot through with pipes and cables, culverts and tunnels, it hums with light, power and information, groaning under the weight of modern living. Yet, up on the surface, we find it easy to ignore what is right under our noses.
Even without human intervention, the Earth bucks and shakes under its own primordial rules, as continents creep, rocks erode and rebuild, and gravity does its worst. Meanwhile, the ground teems with a life of its own. An ocean of micro-organisms stretches down into the depths, into the pores in the rocks, into the very foundations of our islands.
The comparatively fertile shallow layers of soil that we walk over every day provide rich pasture for a hitherto undreamed of array of bacteria, protozoa and fungi, whose existence among the particles of clay, sand, rock and mineral is only today emerging - thanks to the latest techniques in molecular biology. Mites, slugs and beetles burrow through these microscopic communities and, in the UK alone, some 28 species of earthworm - "nature's ploughs" - make fast work of the litter discarded by the plants whose roots festoon this underworld.
Such organisms have had hundreds of millions of years to adapt to a life of unending excavation. By comparison, humans are novices. But our hunger for progress has driven us rapidly to make up for lost time, and then some.
Successive generations have turned the earth into a honeycomb to house the utilities we need to ease our lives up above. We have reshaped it into a vast, slumbering golem whose confined animation keeps our cities alive. And as we have conjured this elemental spirit to serve us, we have gradually obliterated awkward memories of our own more brutish past, once locked safe in the buried archaeological record and now exposed and destroyed by each new trench and cutting.
We have shackled rivers that once ran free, diverting and piping them out of sight. In London, the Tyburn, Fleet, Westbourne and many others have been banished into man-made courses to leave just their names echoing in the uncompromising architecture of the city that rose over them. This great disappearing trick began in the 16th century as the streams became stinking, open sewers that had to be hidden out of sight and, more to the point, out of smell, by impermeable brick vaults. These lost rivers at once spawned legends, as such secret locales always do, such as the 18th-century story of a race of subterranean pigs that roamed the capital's filthy Stygian waterways.
As human populations mushroomed in cities throughout the country, so did the need to draw away the waste they produced with the least fuss. So we dug down, planting vast networks of sewers around the lost rivers that had been conscripted to the cause, taking the strain from the cesspits that once punctuated and polluted every settlement. There was little planning behind these snaking labyrinths and, by the 19th century, they proved hopelessly inadequate to cope with the demands of the emerging industrial society.
In 1858, the year of the "big stink", the stench threatened to overwhelm London and Parliament considered moving upstream to Hampton Court. The following year, the great Victorian engineer Joseph Bazalgette was given the task of saving the city from drowning in its own muck. He looked downwards for a solution: a system of huge parallel sewers, stretching some 450 miles in total, to channel the waste into the Thames, far downstream from the city. Londoners could take the pegs from their noses once more.
But Bazalgette's subterranean network is now well over a century old, its brickwork worn by the passage of so much sewage for so long. It has been patched and amended, but not replaced. Sewers all over the country are in a similar state. Just 389km of the UK's 353,000km of sewers were replaced in 2002, a tiny proportion that suggests a rather unpleasant problem being stored up for future generations to deal with.
These geriatric pipes are now expected to stand strong against a grim new threat to their failing health - the impact of a particularly dangerous aspect of the diet of those who rely on them. While corpulent humans are regularly warned of the harm that fatty foods is doing to our furred-up arteries, the sewers are likewise awash with fats, poured away by our cooks. Restaurants and fast-food outlets regular dispose of cooking residues into the drains. But these fats, oils and grease are too slovenly to reach the treatment plants. They coagulate and stick to the old pipes and brickwork, furring them up, just like our arteries, until the pressure ruptures them and raw sewage gushes into basements and streets. At that point, we tend to take notice.
Southern Water, which manages 13,000 miles of sewers across southern England, recently revealed that more than a third of the 11,000 blockages the company deals with every year were fat related. Thames Water spends pound;7 million annually clearing 100,000 blockages from its 40,000 miles of sewer. It estimated that half of these are caused by fat.
Soho, in the heart of London, has an especially poor prognosis. Last year, Thames Water's "flushers" cleared a solid plug of fat that ran the entire length of a 150-foot pipe in the centre of the restaurant-intensive district. Some 1,000 tons of fat gets poured into the company's sewers each year, with Christmas feasting producing a 25 per cent surge in the amount that goes down the drain.
CCTV, radar and thermal imaging have all been deployed to locate congealed fat in sewers worldwide, and a variety of strategies have been devised to deal with it, including high-pressure hoses and vacuum trucks. A US biotech company has even developed bacteria to break down the grease in order to prevent an urban heart attack.
Fat is not the only threat to our sewers. A sudden downpour in London can result in Bazalgette's put-upon system of storm drains sending raw sewage cascading into the river that runs through the heart of the city. Between April and August last year, more than 12 million cubic metres of toxic raw sewage was discharged into the Thames from overloaded pumping stations. The fat that travels with it forms an evil, sullen grease that stubbornly glues the detritus of city life to everything that floats, while the sewage turns the river oily and flat with its evil content. It takes days to clear. And every time a sewer is breached, the urban rat population that lurks within gets an opportunity to spill out on to the surface, a reminder of the unwelcome "bugs" lurking in our city's veins.
London's water supply, in contrast, is currently in a state approaching rude health, following the recent opening of the London ring main. This is now officially the capital's deepest tunnel system, distributing water from reservoirs across the South-East. A 2.5m diameter pipe circles the metropolis for 50 miles at an average depth of 40 metres below the surface.
While the ring main itself is state-of-the-art, it almost goes without saying that the 8,000 miles of pipework that carry the water to homes and businesses in London are much more modest affairs and mostly considerably older.
The National Grid prefers to eschew the underworld, instead carrying the bulk of its electricity high on pylons. There cannot be many people who, when confronted by a chain gang of these skeletal steel giants plodding over the countryside, would not wish to condemn electricity to the bowels of the earth along with our water.
But the cost of insulating underground cables with layers of treated paper tapes, aluminium and plastic is prohibitive. Air is a much cheaper option.
But within the cities, power has to share the congested ground with all the other utilities, adding real danger to those digging any hole. In the past five years, five people in the UKhave died and 172 have been seriously injured by accidentally making contact with a subterranean power line.
There are also gas pipelines down there, first introduced to the country in 1805 to illuminate a street on George III's birthday. In some cities, there were also networks of hydraulic pipes, dating from the 19th century, which carried pressurised water to power lifts and raise theatre curtains. The steel pipes are still down there, and were in constant use until the late 1970s, though most are abandoned now.
The most recent additions to the twisted tangle over which cities are built have been the telecommunications cables. The old analogue signals between telephone and exchange were mostly carried along copper wire. But in the digital age, slender threads of glass have proliferated beneath the streets, like the tendrils of a vast, global fungus, carrying enormous quantities of information in optical fibres. The networks themselves can be resilient, but breaks do occur. Some 20 different telecommunications companies have installed their own cables below the City of London, all of them perilously close to the surface. A US study hints at the cost of cutting one of those lines, showing that a 36-fibre cable can carry up to 870,912 circuits and generate more than $175,000 per minute in revenue.
However, some of British Telecom's cables are well protected, carried in the deep-level tunnels that were dug under London to protect the telephone network from German bombs. The variety of information these cables can carry, in the form of digitised pulses of light, is practically limitless.
Reaching below our feet could be the cosmic hiss of a supermassive black hole in some distant galaxy - the eMerlin project has just connected Jodrell Bank's iconic radio telescope in Cheshire into a 216km national network via carefully laid optical fibre connections and trunk routes. Or it could be the precise directions to carry out a delicate operation - a team of New York surgeons recently removed a gall bladder from a patient in France using remote-controlled robots.
More prosaic are the underground travellers, commuting in cities like Glasgow and London. Every hour, 150,000 of them enter the Tube, many of them descending into the depths of the earth to travel on the capital's 93 miles of deep-level track. The first London underground lines were laid in covered trenches in 1860 and carried steam locomotives. Today, the electric trains on the Northern Line plunge to some 67.4m below the surface at Hampstead. Seasoned commuters have turned avoiding eye contact with their fellow travellers into an art form. Yet all around them, deep under the ground, are shops and galleries, specially commissioned paintings, prints and photographs by the likes of Man Ray and Graham Sutherland - even poems inside many carriages.
Much of what we choose to bury is obscure, sometimes even secret. There are sensitive sub-atomic particle detectors, designed to unravel the most profound mysteries of nature, secreted in deep-level mines such as one at Boulby in Yorkshire. There are nuclear bomb shelters and battle headquarters from the Cold War right across the UK. And Whitehall is reputed to sit atop a great complex of tunnels and chambers protected by concrete walls 15-foot thick.
The truth is the modern city cannot live without its buried utilities. They are its veins, its arteries, its nerves, its sinews. Cut off the life support and the city will surely die. Like so many of our resources, we havealways taken the ground beneath our feet rather for granted - but now we may need to pay it more attention.