The future will know us through our mines. Unlike many of the changes humanity has wrought on the planet’s surface, which will disappear in geologic time, some of our underground doings have left permanent scars. So argue the authors of a new paper examining such human impacts on Earth.
"The underground realm for most of us is out of sight, out of mind," observes geologist Jan Zalasiewicz of the University of Leicester in England, lead author of the new paper, published July 24 in the journal Anthropocene. "Yet it is seeing significant change that in some ways is as striking as any that humans have made to the Earth's geology, and that is permanent, even geologically."
Zalasiewicz and his co-authors propose a new term for such underground disturbances: anthroturbation. The name derives from the word bioturbation, which refers to the kind of trace left behind in the Earth by animals such as ants or wolves when they dig their homes. Humans take this kind of disturbance to a much deeper level.  
In the present day, while certain tree roots can reach as much as 68 meters deep in the Kalahari Desert of South Africa, miners in that country have tunneled five kilometers below Earth’s surface in pursuit of gold. The world's deepest borehole, in northwestern Russia, plunges more than 12 kilometers into the ground. "The scale is quite unprecedented in past geology," Zalasiewicz says. "Things like mines and boreholes, even compacted by pressure and chemically changed by underground fluids, are big and obvious punctures in the rock. They're not subtle."
Such mines and boreholes are also abundant: there are at least 1 million boreholes in the U.K. alone and at least that many wells currently active in the U.S. Mines leave voids in precise geometric patterns very different from anything seen in normal geology. Least subtle of all may be the subterranean scars from the development of nuclear weapons—not the uranium mines, although those will be there too, but masses of shattered rock atop the radioactive melt core of a bomb test, which can be hundreds of meters across. Just one such test in Alaska created as much melted rock as a medium-sized volcano.
The scientists in charge of the geologic timescale—known as stratigraphers—have used the appearance of animal burrows to define the beginning of the Cambrian period, more than 500 million years ago. In similar fashion, anthroturbation might herald the Anthropocene, a putative new geologic epoch marked by Homo sapiens’ extensive impacts on the planet. Zalasiewicz is the head of the stratigraphy working group attempting to determine whether recognition of the Anthropocene as a true geologic epoch is scientifically justified; the group plans to deliver a recommendation in 2016.
Regardless of any stratigraphic import, such human interference beneath Earth’s surface looks set to continue, whether in the form of underground storage of greenhouse gas pollution or continued mining for the elements necessary to construct modern conveniences above ground. Along with techno-fossils like the subsurface machinery of a modern sewer or subway system in a coastal city, anthroturbation may serve as a distinguishing mark of the human period—as long as it may be—on Earth.
Far in the future, such anthroturbation will look similar to marks from magma oozing through or around other rocks deep underground—known to geologists as igneous intrusions—and it will last as long. "As these traces are made well below the reach of erosion, many will last over geological timescales: for millions of years or even hundreds of millions of years," Zalasiewicz adds. "Some, I would guess, will remain for as long as there is a planet Earth."