It's estimated that humans have altered over half of the planet's surface, and those changes are easy to see — the ice sheets are melting, forests are shrinking and species are going extinct.
People have changed the planet so dramatically that some geologists think the Earth has entered a new phase in its geological timeline, named the "Anthropocene." But what about the marks humans are leaving deep underground?
"Because it's not in our immediate living environment, it doesn't seem as significant," said Jan Zalasiewicz, a senior lecturer in palaeobiology at the University of Leicester. But, as Zalasiewicz and two of his colleagues argue in a new study, human activity below the surface is permanently changing Earth, and a sprawling web of holes from mining and energy exploration provides more evidence that the planet has entered the Anthropocene. [World's Weirdest Geological Formations]
Out of sight, out of mind
The distance to the center of the Earth is roughly 3,960 miles (6,373 kilometers). Animal life stops 1.2 miles (2 kilometers) below the surface — the depth where miners discovered deep-dwelling worms in South African gold mines. All known microbial life stops at a depth of around 1.7 miles (2.7 kilometers). But humans have left a permanent mark well beyond those depths, geologists say.
When an animal dies, it only leaves behind one skeletal record of itself, but the same animal might leave hundreds of so-called trace fossils in the form of burrows. Most animals leave behind trace fossils a few inches deep. The deepest burrowers are Nile crocodiles, which dig dens up to 39 feet (12 meters) deep. The deepest-reaching plant roots belong to the Shepherd's tree in Africa's Kalahari Desert, which can reach 223 feet (68 meters) deep. Humans also leave trace fossils behind, but these can reach as deep as 7.6 miles (12.3 kilometers) and are permanently changing rock layers.
"No other species has penetrated to such depths in the crust, or made such extensive deep subterranean changes," the researchers wrote in the journal Anthropocene.
Humans' first underground foray came during the Bronze Age, when people began digging shallow mines in search of flint and metals. The Industrial Revolution of the 1800s sent humans even deeper below the surface. Still, many of the disturbances, like water wells, sewage systems and subway lines, were relatively shallow and extended less than 330 feet (100 meters) below the surface. Only after 1950, a period referred to as the "Great Acceleration" by some geologists, did humans really plunge below 330 feet, Zalasiewicz and his colleagues explained.
Growing demand for resources led to more mining to collect coal and other minerals. In most cases, mining only extends several hundred feet deep, but gold mines in South Africa reach almost 3 miles (5 kilometers) below the surface.
In search of riches
More and more boreholes have also appeared over the last several decades. Some boreholes are drilled to harvest geothermal energy. But others are used to pull out natural material from the Earth, such as hydrocarbons, natural gas and ores.
If all the world's oil boreholes were stacked on top of each other, they would span over 31 million miles (50 million kilometers). That's almost as far as the minimum distance between Earth and Mars. Or, to put another way, for every human on Earth, there are about 23 feet (7 meters) of boreholes. Oil pulled from deep boreholes is often replaced with water that seeps in from neighboring rocks or with carbon dioxide that is pumped in during a process called carbon sequestration. [Top 10 Ways to Destroy Earth]
The web of mines and boreholes "arguably has the highest long-term preservation potential of anything made by humans."
At 7.6 miles (12.3 kilometers) long, the Kola Superdeep Borehole in Russia is the deepest hole in the Earth's surface made by humans. It was drilled in northwestern Russia in the 1980s as part of a scientific investigation.
Underground nuclear tests have also left their mark, the researchers note. Test sites often contain broken-up and melted underground rocks and disturbed water tables. Huge underground caverns hold stored radioactive waste from the tests.
These human-made changes below the surface will stay there, protected from the natural erosion and weathering that happens above the surface. The web of mines and boreholes "arguably has the highest long-term preservation potential of anything made by humans," Zalasiewicz and his team of researchers wrote. The scientists estimate it will take millions of years for weathering and erosion to uncover tunnels just a few miles below the surface.
— Kelly Dickerson, LiveScience
This is a condensed version of a report from LiveScience. Read the full report. Follow Kelly Dickerson on Twitter. Follow LiveScience on Twitter, Facebook and Google+.
FAQs
Researchers say our global web of mines and boreholes, going as deep as 7.6 miles underground, may be humanity's longest-lasting mark on the planet.
How deep have humans gone underground? ›
The 23-centimetre-diameter (9 in) boreholes were drilled by branching from a central hole. The deepest reached 12,262 metres (40,230 ft; 7.619 mi) in 1989, the deepest human-made hole on Earth, and remains so as of 2024.
What is the deepest hole ever dug in the earth? ›
With a depth of 12,262 metres, the Kola Superdeep Borehole in north-west Russia is the world's deepest human-made hole.
What type of animals can be found at 39 feet underground? ›
The deepest tunnelers among animals—crocodiles that can burrow 39 feet down—cannot compete with us at all. Humans have traveled, in the deepest mine in the world, almost 2.5 miles underground (to say nothing of our boreholes, which go nearly three times deeper).
How deep can we dig? ›
Deepest drillings
The Kola Superdeep Borehole on the Kola peninsula of Russia reached 12,262 metres (40,230 ft) and is the deepest penetration of the Earth's solid surface. The German Continental Deep Drilling Program at 9.1 kilometres (5.7 mi) has shown the earth crust to be mostly porous.
Why did Russia stop drilling the Kola Superdeep Borehole? ›
Despite the significant depth achieved, the drilling faced challenges like increasing temperatures and rock densities, leading to the project's discontinuation in 1992, with the hole being sealed in 2005.
Has anyone reached Earth's core? ›
Although the scientific community has made clear that this is proven false, the idea nevertheless is a less popular feature of many fantasy and science fiction stories and of some conspiracy theories.
Why haven't we been able to drill farther? ›
To make things more difficult, as they drill deeper into the Earth, they'll encounter extreme temperatures, possibly in excess of 1,000 degrees Fahrenheit (538 degrees Celsius), and fantastic amounts of pressure — as much as 4 million pounds per square foot in the vicinity of the mantle.
Have humans dug to the mantle? ›
Pieces of Earth's rocky mantle were collected by a drilling vessel called the JOIDES Resolution about a mile beneath the North Atlantic ocean floor at an underwater mountain called Atlantis Massif – located on a special “tectonic window” of the Mid-Atlantic Ridge.
Have we drilled into the mantle? ›
At an underwater mountain in the middle of the Atlantic Ocean, scientists have drilled nearly a mile beneath the ocean floor and pulled up an unprecedented scientific bounty — pieces of Earth's rocky mantle.
People throughout history have temporarily lived below the surface for various reasons. If there were no materials to build houses with, they dug subterranean homes, Hunt told Live Science. In places with extreme climates, people went beneath the earth in the summer to stay cool and in the winter to stay warm.
How deep do crocodiles burrow? ›
Most animals leave behind trace fossils a few inches deep. The deepest burrowers are Nile crocodiles, which dig dens up to 39 feet (12 meters) deep.
What animal lives the deepest on Earth? ›
Species evolved to withstand heat and crushing pressure. A "devil worm" has been discovered miles under the Earth—the deepest-living animal ever found, a new study says.
How hot is the Earth 1 mile down? ›
It would be hot underground. Geothermal gradient indicates that on Earth, 1 mile underground would be about 40-45 C (75-80F, just as you said) hotter than on the surface. Unless your underground city lies under permafrost, that would be a definite challenge for human habitation. Ventilation would be a challenge too.
Why can't we drill to Earth's core? ›
The pressure in the Earth's core is more than 3,000 times the pressure at the bottom of our deepest ocean. The temperature is more than 5,000°C. Your poor little tunnelling machine would be crushed to a pea and then cooked to a bubble of gas long before it could get anywhere near Earth's core.
Why can't we drill into the mantle? ›
There are places where the mantle is very shallow, for example in oceanic spreading ridges, but those are active magmatic zones and drilling through liquid hot magma is not something that's feasible.
Why can't we dig deeper into the Earth? ›
The main reason why we can't dig deeper than 12.2 km into the Earth is due to the extremely high temperatures and pressures that exist at deeper depths. As we move deeper into the Earth, the temperature and pressure increase steadily.
How long would it take to fall down the Kola Superdeep Borehole? ›
In diameter, the Kola Superdeep Borehole measures 9 inches diameter. It features a metal lid welded on it, so it is unlikely that anyone would ever fall down. However, should someone fall down the hole, it would take around 3.5 to 4 minutes to reach the bottom.
How long did it take to dig the deepest hole? ›
The world's deepest man-made hole, the Kola Superdeep Hole is in Russia. This is 12,262m deep, was completed in 1989 and took 20 years to dig. They stopped digging when the heat and pressure made drilling really difficult.
Where is the deepest hole humans have ever dug? ›
The Kola super hole at 12.345km remains the deepest point drilled into earth till now. The Russians who bored the hole stopped it because the temperature at that point reached 356 degrees Fahrenheit (180 degrees centigrade) and there was equipment breakdown.
