Worlds of David Darling > Children's
Encyclopedia of Science > Could You Ever Dig a Hole to China? > 3. Scratching
COULD YOU EVER DIG A HOLE TO CHINA?
a book in the Could You Ever? series by David Darling
3. Scratching the Surface
There are two ways to go down into the crust. You can dig your own hole,
or use one that nature has already provided. Caves in many parts of the
world offer the chance to explore a fantastic, silent realm beneath the
surface. This is the realm of weird rock formations, of beautiful crystals,
and of strange creatures that have never seen the light of day. Caves provided
the first means by which human beings could enter the interior of their
|For thousands of years, people have entered
the Earth's interior through caves such as this one on the Malaysian
island of Sarawak
Because of the endless movement of different parts of the Earth’s crust,
some places that were once covered by ocean have been lifted up to form
land. This explains why fossils of sea creatures can often be found in rocks
hundreds of miles from the nearest shore. These creatures died millions
of years ago. At that time, the Earth’s oceans and continents were arranged
quite differently than they are today.
Most of the animals and plants that died in the sea long ago are not well
preserved as fossils. Instead, their bodies were completely crushed as layer
upon layer of other remains, or sediment, piled up on top of them. Gradually,
the broken shells and skeletons of the dead creatures turned into rock.
One common rock that formed in this way is LIMESTONE.
The Story of Caves
The great earth movements that raised up the limestone from ancient seabeds
also caused it to crack. Rainwater could trickle down into the cracks and
easily work its way deep below the surface. But first, the rain had to soak
its way through layers of soil above the limestone. In doing so, it mixed
with carbon dioxide, a gas produced by dead and decaying plants. Carbon
dioxide picked up from the soil, and also from the air, turns pure rainwater
into a weak acid known as carbonic acid.
|Aragonite Alcove in Carlsbad Caverns, New
Mexico, is known for its icicle-like stalactites and stalagmites.
These are slowly formed from drops of calcium carbonate
As rainwater penetrated the limestone cracks, the acid in it dissolved the
limestone and carried it away. Over many thousands of years, the cracks
grew wider and wider, until they became caves.
In every state in the United States and in many countries around the world,
there are caves that were created in this way. Some, like Carlsbad Caverns
in New Mexico and Mammoth Cave in Kentucky, have sections that have been
made safe for anyone to visit. The part of Mammoth Cave that is open to
the public is just the start of a huge series of passageways and caverns
that is known to extend for more than 330 miles. That makes it the longest
in the world. But although caves may be very long, they do not penetrate
very deeply into the Earth. In fact, the deepest known point in any natural
cave is just 5,036 feet – less than a mile beneath the surface.
Caves may seem unlikely homes for animals and plants but, in fact,
they are inhabited by a wide range of species. Some creatures, such
as the insect-eating bats of Carlsbad Caverns in New Mexico, spend
only part of their time underground. Other species have become so
well adapted to living below the surface in complete darkness that
they could not survive on the surface. These permanent cave dwellers
are called troglobites.
Because troglobites live where there is absolutely no light, they
have no need of eyes. Instead, they find their way mainly by feel
and smell. They also lack any pigment, or skin coloring. In surface-dwelling
species, such pigment helps block out harmful rays from the Sun. As
a result, troglobites are either white or, if their blood shows through
their skin, pale pink.
Blind cave fish, salamanders, crayfish, spiders, mites, and certain
kinds of insects are among the animals found underground. All of these
species have some features in common with their relatives that live
on the surface. But they have also adapted in special ways. Most insects,
for instance, have amazingly long feelers which they use for tracking
down prey in the dark.
Tunnels and Subways
Nature takes many thousands of years to wear away rock to form caves. But
today, human beings can dig tunnels, miles in length, at a far greater speed.
|Commuters board a subway in
a Washington, D.C., metro station
As early as the mid-nineteenth century, some cities were becoming so crowded
that plans were made to build underground railways. The first one to open
was the Metropolitan Line in London in 1863. Today, the London Underground
is the largest subway system in the world. At its deepest point, it is 102
feet below street level.
To begin with, subway tunnels were built by "cut and cover" methods. This
involved digging a trench along the street, lining it with brick, and then
covering it with brick arches or girders to make a roof. Later, tunnels
were bored through the ground by workers using spades and pickaxes, and
then lined with cast iron.
In recent years, giant machines have replaced most human labor. Many major
underground projects now use tunnel-boring machines, or TBMs, equipped with
rotating cutting teeth. As the rock breaks up at the front of the TBM, it
is carried away on conveyor belts to be loaded onto waiting trucks or trains.
The world's longest undersea tunnel is the Channel Tunnel, which runs 130
feet beneath the English Channel and extends 23½ miles between Dover
in England and Calais in France. Giant TBMs, 886 feet long with cutting
heads 15.7 feet wide, bored through the soft chalk at a rate of five-eighths
of a mile a month to make the Channel Tunnel, which now allows high-speed
trains to travel between London and Paris in just three hours.
Riches of the Earth
Humans dig holes for many reasons other than underground transport. Mining
is one of the most important of these. The Earth is a vast storehouse of
energy and minerals. It can provide us with everything from fuel for warmth
to metals and chemicals for industry.
|Composition of the Earth's
Around 300 million years ago, large areas of the land were covered by steamy
swamps where giant, fernlike plants and other strange vegetation grew. As
these plants died, they were buried deeper and deeper below the surface.
Gradually, they turned into hard, black coal. Burned as a fuel in homes
and power stations today, coal is what remains of vast forests that existed
on Earth before the age of dinosaurs.
In addition to coal, our planet holds huge stocks of various metals. Iron,
for instance, makes up about a twentieth of the Earth's crust and is mined
in large quantities. Like most metals, it is not found in a pure state but
as an ore, combined with other substances from which it must be separated.
Aluminum ore, known as bauxite, and copper ores are also heavily mined for
To find ores and other useful substances in the Earth, prospectors set off
underground explosions which send shock waves through the surrounding rock
and soil. These waves pass through different materials at different speeds.
They are also deflected in various ways. The echoes are picked up by microphones
on the surface. Scientists use this information to construct a picture of
the types of rock, soil, and minerals in that area. If this type of survey
shows that minerals are present in large amounts, mining companies will
set up the machinery to extract them.
Down the Mines
The way in which coal and most minerals are removed from the ground depends
on the size, shape, and location of the deposit. When minerals lie close
to the surface, workers remove the vegetation and top layers of soil and
rock, called the overburden, so that the ore can be dug out. This method
is known as open pit mining. Since the work is all above ground, huge excavators
on crawler tracks and other heavy vehicles are used. Such mining methods,
though, can cause severe damage to the area where they tear up the soil.
Deep mining is far more difficult, dangerous, and expensive. Shafts, tunnels,
lighting, ventilation, and water pumps are all necessary parts of the operation.
Once a shaft has been dug, timber, steel, or concrete pillars are used to
support the rock and protect the workers. Finally, the ore or rock at the
exposed face has to be broken down into small chunks so that it can be moved
to the surface.
The depth of mines varies greatly. Deepest of all is the Western Deep Levels
gold mine in Carltonville, South Africa. There miners work as far as 12,394
feet, or almost two and a half miles, below the surface. This is the deepest
that human beings have ever descended into the Earth. Even though the bottom
of the Western Deep Levels mine is much less than a thousandth of the way
to the center of the Earth, the conditions inside it are uncomfortable.
The pressure of the layers of rock above is tremendous and miners must work
in temperatures as high as 131°F. Still, mines are far from being the
deepest of holes made by human beings.