We're accustomed to thinking geologic history has an extremely long, slow time scale. But in recent years Nobel Laureate atmospheric researcher Paul Crutzen and other scientists have proposed that human activity is changing the planet pervasively and permanently. As a result, they suggest, we've said goodbye to the Holocene epoch, which began 11,700 years ago at the end of the last Ice Age, and entered into a new, man-made Anthropocene epoch.
In a newly published study, Earth scientists Jan Zalasiewicz and Mark Williams, of the University of Leicester, and Colin Waters, of the British Geological Survey, bolster the case for an Anthropocene epoch by pointing out that human activities -- ranging from fracking for oil and gas to underground nuclear tests -- actually have created profound changes deep beneath the Earth's surface.
"Many of these underground transformations, being beyond the reach of surface erosion, will effectively last forever," Williams said in a release. "They can be preserved for millions and even billions of years into the future, and thus may form our most enduring -- and most puzzling -- legacy, for any intelligent creatures that may inherit the Earth from us."
The worry is that such changes may combine with climate change and chemical alteration of the oceans to create an environment that's less and less hospitable to life, including humans.
Here are six ways in which humans have changed the planet beneath its surface.
According to Mining Technology, an industry publication, there are 10 mines that penetrate 2.5 kilometers (1.55 miles) or deeper into the Earth, and the deepest is the Mponeng gold mine in South Africa, which works areas as deep as 2.4 miles (3.9 kilometers). Most mines, like the Japanese gold mine shown above, don't go quite that deep. But according to Zalasiewicz, Williams and Waters, when minerals and other material are extracted from deep in the Earth, the resulting void collapses and leaves a fragmented layer that may cause subsidence of the ground surface above.
A North Sea oil platform
The deepest shaft ever drilled is the Kola Superdeep Borehole, an effort by Soviet researchers from the 1970s through the 1990s, which reaches 7.6 miles (12.26 kilometers) into the Earth. But recently, Russia's Sakhalin-1 offshore oil installation exceeded that depth, reaching 7.9 miles (12.7 kilometers). Deep drilling can change the fabric of the deep Earth, by allowing mud to infiltrate porous rock, and additional arrays of low-angle or horizontal holes can further destabilize the underground.
Increasingly, storage facilities are being created under the Earth's surface, either by excavation or use of existing caverns that were created from mining operations. In Carlsbad, N.M., half a mile below the desert surface, the U.S. Department of Energy is carving out football-field sized rooms to store barrels and boxes of plutonium waste from nuclear weapons manufacturing in a salt formation that officials say will provide a natural sealant for millions of years, according to a New York Times report.
Of the more than 2,000 nuclear explosions since 1945, about three quarters involved test devices being detonated underground, which makes it possible to control the radiation's spread, at least in theory. In practice, one 1970 U.S. underground nuclear test shattered the rock below and released a cloud nearly 10,000 feet into the air, according to CNN. Since 1998, only North Korea has continued to conduct such tests, including a 2013 blast that created the equivalent of a 4.9 magnitude earthquake.
Urban areas, which cover about 3 percent of the Earth's land surface, are honeycombed with increasingly complex networks of tunnels. Some contain sewage, electricity and gas delivery systems, while others provide transportation, such as the Washington Metro subway system shown in this picture. These extensive tunnel systems are likely to last far longer than the structures that humans build above ground, researchers say.
According to an industry report, there are tens of thousands of miles of pipelines -- most of them either under underground or laid on the sea floor -- that transport oil and natural gas. Structures such as the British gas pipeline shown above are considered safer ways to transport fuels than rail or road tankers. One natural gas pipeline in China stretches more than 5,000 miles. The proposed Keystone XL oil pipeline would reach roughly 1,200 miles from Canada to Nebraska.
Most of it would be buried 4 feet underground on land, but some sections would require digging tunnels underneath the Missouri, Platte, Niobrara and 11 other rivers, according to Popular Mechanics. Critics have warned that leaks from Keystone XL might endanger the Ogallala Aquifer -- a key freshwater source for the Great Plains -- although a 2014 federal environmental review said that risks to groundwater were limited because most of the aquifers were several hundred feet below the pipeline. In 2010, a leak to a pipeline in Michigan blackened a 36-mile stretch of the Kalamazoo River.