Final answer:
Subsurface mining is used when mineral resources are too deep for surface mining, involving methods such as shaft and drift mining to access resources like thermal energy, oil, gas, and other minerals. Depletion of resources near the surface has led to an increased reliance on these techniques.
Step-by-step explanation:
When a mineral resource is too deep to be surface mined, a technique known as subsurface mining is used. This includes various methods such as shaft mining, where a vertical tunnel is dug down to access mineral resources, or drift mining, which involves horizontal tunnels. For instance, in geothermal heating, boreholes may be drilled to extract thermal energy from deep within the Earth's crust. Yet this method can lead to depletion of the thermal resource over time. Similarly, for oil and gas deposits, as well as other valuable minerals like gold and salt, drilling and other subsurface techniques are necessary when resources are not accessible by surface mining methods.
Historically, easier resources to access were used first, and as these have become depleted, the need for subsurface mining has grown. Despite the challenges and environmental impact, such as the case with mountaintop mining (MTM) for coal, these methods have become essential for continuing to procure valuable resources hidden deep within the Earth.