TRAVEL back in time and you will find that humankind has always dreamt of travelling though the air, under the sea and beneath the ground. Writers of all eras have imagined fantastic voyages to kingdoms in the clouds, at the bottom of the sea and at the roots of mountains.
Water was the first to be conquered when weighted diving bells came into use in medieval times. Then came air, with the ascents of the Montgolfiers in 1783. True underwater and aerial travel came later. The first underwater voyage was made in the US in 1878 when the Irish emigrant John Holland took an hour鈥檚 trip in his iron submarine, Holland I. Then, in 1903, the Wright brothers made their first powered flight in a heavier-than-air machine.
But what of travelling underground? Back in the last century, some of the first science fiction writers described great iron vehicles with noses like pointed drills that could burrow through the ground, or take us on a fantastic voyage to the centre of the Earth. Some even imagined armies of these steel monsters locked in ghastly battles deep underground. Their descendants live on in the popular 1970s television series Thunderbirds. One of their international rescue craft is the Mole, a nuclear powered vehicle that can drill its way through the ground at fantastic speeds.
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Alas, despite these dreams, a machine that can truly imitate the mole has never been built. The closest we have got is to copy the shipworm. In 1816, Marc Isambard Brunel spent an afternoon in a shipyard watching a worm work its way through a piece of wood. He saw that the worm cut into the wood with the hard, horny projections on its head, and then inched forward by pushing against a coating that it secreted on the tunnel around it.
These observations inspired Brunel to patent the world鈥檚 first tunnelling machine. A shield pressed up against the tunnel face while an iron box behind protected the workers and provided leverage for jacks that pushed the shield forward.
Nowadays, high powered drills and cutters would be mounted on that face. With them, we can just about inch our way along underground. The current world record for underground travel was set by Thames Water鈥檚 engineers in 1992 when digging the London Ring Main. They tunnelled 501 metres in five days, using a 55-tonne cutting machine named Dorothy.
That鈥檚 still far from the achievements of the garden mole. Working flat-out, a mole digging 5 to 10 centimetres beneath a nice, well-drained lawn can reach speeds of 900 metres an hour. The closest we can get to such underground travel is by proxy, with small, remote-controlled tunnelling machines used for laying pipes and cables.
The Rotamole, developed by British Gas, is one such machine. It looks like a short piece of pipe with a sloping chisel-like face. Once down a starting hole, compressed air is supplied from above ground to drive a piston back and forth inside the pipe, making it hammer its way through the earth. At the same time, its face is rotated by a drill string going back up the hole. The Rotamole can be tracked and steered from above ground and can travel as far as 150 metres. But even in the right soil, it cannot beat the garden mole, clocking up speeds of only 35 metres an hour.
Sending a small robot underground, or advancing at the leisurely pace of a 5-tonne Dorothy, does not quite live up to the dreams of Victorian science fiction writers. Nevertheless, we can explore the underground in our imagination or, enter the spaces that humans and machines have slowly tunnelled and dug out underground. Of these there turn out to be a surprising number and variety.
Underneath a modern city is layer after layer of subterranean life, populated by pipes and people, cables and machines, and cockroaches and rats. In the pages that follow, we explore the world beneath cities, travel the sewers in search of alligators, tour crypts to chart the progress of life after death, and also visit underground towns and talk to one of the last well-diggers. Until the nuclear-powered Mole is actually built, this is as far as anyone can go.