No more than a pinprick

For years, the drill bit has laboriously hammered and twisted its way into the hard earth’s crust. Time and again it got stuck. Now the press spokesman for the deep drilling programme in Windischeschenbach announced the end of the scientific project: on 12 October 1994, the drilling rig and its measuring equipment had to be shut down at a depth of exactly 9101 metres and a temperature of 265 degrees Celsius. The reason: the coffers of the research project were empty. The drilling programme was very successful overall, but a continuation was too costly.

The drilling at the Windischeschenbach site near Weiden in the Upper Palatinate was started in 1987 to investigate the earth’s crust and the processes taking place in it more closely. Originally, the geologists wanted to drill to a depth of 14 kilometres. According to their calculations, by then the electronic equipment would have withstood the high temperatures of an estimated 300 degrees Celsius.

The deepest hole in the world

Kola, a Scandinavian peninsula on the icy north-western edge of Russia. Here, where hardly a soul strays, the earth’s crust is over three billion years old. Such old crust is rare, and so a scientific drilling started in 1970. Researchers wanted to bring rock samples from the Earth’s interior to the surface. But at a depth of a good 12 kilometres at a temperature of almost 200 degrees Celsius, the drilling equipment became soft and the electronics failed. The Russian deep drilling programme had to be stopped in 1989. But with its 12262 metres, it is still the deepest borehole in the world today. Over 45,000 rock samples were taken from the earth’s crust during this time. Their exploration will take decades.

Soviet Union digs deepest hole on earth

Never before has a human being penetrated deeper into the earth: On the Kola Peninsula, a Soviet research team drilled a hole more than 12 kilometres deep into the earth’s crust. Because of the unexpectedly high heat underground, the operation was stopped after 12,262 metres.

The former Soviet Union had already begun drilling on the Kola Peninsula in the north of the country in 1970. The aim of the “super-deep Kola well SG-3” was to reach the boundary between the Earth’s crust and mantle and to take rock samples. Kola was chosen because the rock here is more than two billion years old. This is where the drilling machine “Uralmasch-4E” came in, which was also used for oil drilling. Later, it was replaced by a device that would reach up to 15 kilometres into the earth.

On the rocky way underground, 45,000 rock samples were taken, various fossils were discovered and even gold was found. Copper and nickel deposits useful for industry were also located. The biggest surprise, however, was that from a depth of 10 kilometres, the heat was unimaginable. At 180 degrees Celsius, the temperatures were much higher than expected. Finally, at a depth of 12,262 metres, the journey came to an end. Technical breakdowns prevented the drilling work from progressing. There is even talk of infernal noises underground. Is the ghastly sound a horror story or is the earth’s crust groaning? In any case, the super-deep Kola borehole still poses some riddles for science.

USA depth record broken

With the Kola well, the Soviet Union was pursuing another interest besides a scientific one:

They wanted to outdo the USA, which had reached a depth of 9,583 metres with its Berta Rogers well. This borehole, which was considered the deepest in the world from 1974 onwards, is located in the US state of Oklahoma. But the record only lasted five years. On 6 June 1979, the USSR broke the Americans’ depth record with the Kola well. Despite all the records: with an earth radius of 6371 kilometres, both holes are just a small blip on the earth’s surface.

Daredevil drilling on the high seas

Near the Mexican Pacific coast, four propellers fight against the waves. They try to hold the drilling ship, the Cuss I, in place. For 3500 metres below it, its drilling rig is supposed to turn into the seabed. After several weeks and persistent attempts, the US researchers have now succeeded in drilling a 183-metre deep hole.

Oceanic crust is much thinner than continental crust. This is why it was originally believed that at a depth of 10 to 15 kilometres they would be able to hit the boundary to the Earth’s mantle – the so-called Moho boundary. With their 183 metres, the researchers did not get very far, but the project is nevertheless a success: it shows that deep drilling on the high seas is possible. It will only be a matter of time before it is possible to reach the Earth’s mantle.

The discovery of the Moho border

In 1909, the earth shook near the Croatian city of Zagreb. Andrija S. Mohorovičić – a Croatian scientist who researched the inner structure of the earth – took a closer look at the records of the earthquake waves. After many calculations, he found that some waves are refracted at a denser layer of rock about 30 to 50 kilometres below the Earth’s surface – as if they were hitting a hard rock. He thus discovered the transition between the Earth’s crust and mantle: the “Moho boundary”, which was named after him.

Why is the earth warm inside?

The liquid interior of the earth bubbles beneath our feet. Volcanic eruptions and geysers show the heat that prevails there – over 6000 degrees Celsius in the earth’s core. But why is it actually so hot in the earth?

Much of the heat still comes from the childhood days of the Earth, when dust and rock fragments condensed to form a planet. However, the word “compact” sounds a bit too harmless: In reality, you have to imagine it like many large meteorite impacts – each impact a gigantic explosion that heated up the young planet and melted the material.

Since then, things have calmed down a bit and the Earth is cooling down again. However, it is doing so extremely slowly, as the heat in the Earth’s interior can only escape very slowly into space. Hot magma flows in the Earth’s tough mantle transport the heat upwards. There it remains trapped under the rigid earth’s crust as if under a lid. Only slowly does the crustal rock release the heat into space.

In addition, heat is still being produced inside the Earth. This is because the earth has a lot of radioactive substances in its core, such as uranium. Since the formation of our planet, they have decayed and given off heat over a very long period of time. This “fuel” will last for many billions of years.

The outermost shell of the earth

Like an egg from an eggshell, the Earth is also surrounded by a hard shell. This outermost layer surrounds the Earth’s mantle and is called the Earth’s crust. If you compare the earth to a peach, the earth’s crust is – relatively speaking – as thick as its skin. Under continents, it reaches an average depth of 40 kilometres, under the oceans even only about seven kilometres.

Below this lies the outer part of the Earth’s mantle, which reaches down to a depth of about 100 kilometres. It is also solid, but consists of heavier rock. The earth’s crust and this outermost part of the mantle together are also called the “lithosphere”. This solid layer of rock is broken into plates of different sizes, which drift very slowly on the hot, viscous mantle of the Earth.

Where the molten rock from the hot mantle penetrates upwards, the earth’s crust can break open. Lava then flows out and becomes new crust. This mainly happens where the plates of the lithosphere adjoin each other, such as at the mid-ocean ridges.

In Iceland, for example, these plate boundaries are clearly visible: Cracks and furrows run through the earth’s crust here, where the Eurasian and North American plates drift away from each other. There is also a plate boundary in the Mediterranean region. Because the African plate is pressing against the Eurasian plate here, there are many volcanoes and earthquakes in Italy.

The crust is covered by the soil. The soil of the land masses forms from weathered rock and the remains of animals and plants. The seabed, on the other hand, develops from deposits such as clay and the sunken remains of marine organisms. On the coasts, the seabed also consists of deposited debris that has been eroded from the mainland and washed into the sea.

How is the earth constructed?

In the beginning, the young Earth was a hot ball of molten matter. All the components were initially well mixed, just as they were distributed when the earth was formed: Metals, rocks, trapped water and gases and much more – a big mess.

But over time this changed: the heavier substances sank downwards to the centre of the earth – especially metals. Rocks, on the other hand, were somewhat lighter and rose upwards, the lightest ones to the earth’s surface. There they slowly cooled and solidified.

This is how the earth’s material separated into the three spherical layers we know today. You can imagine the structure of the Earth like a peach: on the outside is a wafer-thin “shell” of light, solid rock – the Earth’s crust. On average, it is only 35 kilometres thick.

Beneath the crust is the “flesh” – the almost 3000 kilometre thick mantle of heavy viscous rock. And inside the earth lies the earth’s core made of the metals iron and nickel.

The Earth’s core itself initially consists of an outer layer about 2200 kilometres thick, the outer core. It is over 5000 degrees Celsius hot there, so the metal is molten and as thin as mercury.

On the very inside is the inner core, somewhat smaller than the moon. At over 6000 degrees Celsius, it is even hotter than the outer core – but surprisingly solid. This is because with increasing depth, not only the temperature rises, but also the pressure. The outer layers that weigh on the Earth’s core compress its material so unimaginably strongly that it cannot liquefy.

How do we know how the earth is constructed?

We can fly to the moon, but a journey to the centre of the earth will always remain science fiction. At a depth of just a few kilometres, any drilling rig becomes soft because it cannot withstand the enormous pressure and high temperature. Nevertheless, researchers know very precisely how the earth is structured – but how?

Similar to an X-ray machine, geologists can look inside the earth without having to cut it open. Their “X-rays” are earthquake waves: When there is a strong tremor in one place, the vibrations spread through the entire body of the earth, similar to sound waves in the air.

However, these waves do not always travel at the same speed: in dense and hard material, the vibrations are transmitted faster than in lighter and softer material. If they encounter a layer of rock with a higher density, they can also be refracted or reflected back, like rays of light on a pane of glass. And some waves can only travel in solid or viscous materials and cannot pass through liquids at all.

When the earthquake waves finally arrive on the other side of the world, they are recorded by a worldwide network of highly sensitive measuring devices called seismographs. From the patterns in these diagrams, researchers can read the type of waves and their speed, and trace the waves’ path through the globe.

In this way, the researchers learn a lot about the Earth’s interior – for example, at what depth there are layers of rock or metal and whether they are solid, viscous or thin.