A subsurface activity is any process, natural or human-made, that takes place beneath the Earth’s surface. Mining, hydraulic fracturing, magma movement, tunneling, and geothermal energy extraction all qualify. If you encountered this question on a test or quiz, the correct answer is whichever option describes something happening underground rather than on the surface or in the atmosphere.
The term covers a surprisingly wide range of activities, from tectonic plates grinding miles beneath your feet to engineers drilling wells for oil. Here’s a closer look at the major categories.
Natural Geological Processes
The most powerful subsurface activities are the ones humans have no control over. Tectonic plate movement is the big one: Earth’s outer shell is broken into massive plates that slide, collide, and pull apart. Where plates converge, one can dive beneath the other in a process called subduction, dragging roughly 5 to 6 kilometers of ocean crust and sediment down into the mantle. The collision of India’s plate underthrusting beneath Asia is what built the Himalayas and continues to this day.
Magma movement is another major subsurface activity. Along mid-ocean ridges, partially melted rock rises from the mantle to create new ocean floor. Farther from these ridges, volcanic islands tend to form from deeper magma sources. At subduction zones, the sinking plate generates a completely different type of volcanic activity, feeding chains of volcanoes like those along the Pacific Ring of Fire.
Mining and Drilling
Underground mining is one of the oldest human subsurface activities. Workers extract coal, gold, copper, and other minerals from tunnels and shafts that can reach remarkable depths. South Africa’s gold mines extend more than 3.5 kilometers below the surface, making them among the deepest working environments on Earth.
Oil and gas drilling pushes even deeper. The deepest hole ever drilled is the Kola Superdeep Borehole on Russia’s Kola Peninsula, which reached just over 12 kilometers (about 8 miles) before the project was abandoned in 1989 when the drill became stuck. Within the United States, the Bertha Rogers gas well in Oklahoma holds the record at roughly 9.7 kilometers (6 miles).
Hydraulic Fracturing
Hydraulic fracturing, commonly called fracking, is a subsurface extraction method used to pull natural gas and oil from deep shale rock. Operators drill wells, often horizontally, then pump in a high-pressure mixture of water, sand, and chemicals. This cracks the shale and releases trapped gas or oil, which flows back up the well for collection. In states like Colorado, Oklahoma, Pennsylvania, and Texas, fracking operations sometimes sit close to residential communities, which has made the technique a focus of environmental and public health debate.
Geothermal Energy Extraction
Geothermal energy is generated by tapping heat stored in underground rock. A working geothermal system needs three things: hot rock, fluid to carry the heat upward, and pathways (natural fractures) that let that fluid move through the rock. Wells bring the heated fluid or steam to the surface, where it drives turbines to produce electricity. The cooled fluid is then reinjected underground to be heated again.
Not every location has the right natural conditions, so engineers have developed enhanced geothermal systems that create artificial reservoirs. They inject fluid into hot rock to open up or create fractures, essentially building the plumbing that nature didn’t provide. A newer approach, closed-loop geothermal, skips the fracture network entirely and circulates fluid through sealed underground pipes, functioning like a buried radiator. The most ambitious frontier is superhot geothermal, which targets rock above 375°C, where water enters a state with properties of both liquid and gas, capable of carrying enormous amounts of energy.
Carbon Storage Underground
Pumping captured carbon dioxide into deep geological formations is a growing subsurface activity aimed at reducing greenhouse gas emissions. The U.S. Department of Energy is investigating five types of underground formations for this purpose: saline formations, oil and gas reservoirs, unmineable coal seams, organic-rich shales, and basalt formations. The CO2 needs to be injected below about 800 meters (2,600 feet), where natural temperature and pressure keep it in a supercritical state, meaning it behaves like a dense fluid that won’t easily escape back to the surface.
Tunneling and Underground Infrastructure
Subway systems, utility tunnels, sewage networks, and underground power cables all require subsurface construction. Modern cities rely on complex buried infrastructure for water, electricity, and transportation. Building these systems involves tunneling, pipe-jacking (pushing prefabricated pipe segments through the ground), and trenching, all of which are subsurface activities with significant engineering and geotechnical challenges.
Microbial Life in the Deep Subsurface
Subsurface activity isn’t limited to geology and engineering. An entire biosphere exists deep underground, populated by microorganisms that survive without sunlight. These communities are powered primarily by chemical energy, with hydrogen gas serving as a key fuel. Deep subsurface microbes use hydrogen to drive processes like generating methane, reducing sulfate, and reducing iron.
One standout organism, discovered in a South African gold mine and named Candidatus Desulforudis audaxviator (“bold traveler in search of sulfur”), is completely self-sufficient. It can fix its own carbon and nitrogen, synthesize all its own amino acids, and switch between feeding on organic matter and producing its own food depending on conditions. Its hydrogen supply comes partly from the radioactive decay of uranium in surrounding rock. Fungi have been detected in bedrock fracture water in Finland at depths of 300 to 800 meters, and tiny roundworms have been found nearly 3.6 kilometers deep in a South African gold mine, likely feeding on the bacteria around them.
How Subsurface Activity Is Detected
Scientists and engineers use several non-destructive methods to monitor what’s happening underground. Ground-penetrating radar and seismic reflection send waves into the earth and analyze how they bounce back, producing high-resolution images of near-surface features like cavities and rock layers. Electromagnetic methods, such as electrical resistivity tomography, penetrate deeper but can be disrupted by external interference. Gravity-based methods detect differences in underground density to locate buried voids or structures. The newest tool, the quantum gravimeter, uses atomic physics for extreme precision, though it remains expensive and difficult to deploy in the field.