What is the primary location where coal mining occurs?

Coal mining primarily occurs underground, which is why this answer is correct. Underground mining techniques are employed to extract coal from deep within the earth, where thick seams of coal are situated beneath layers of rock and soil. This method allows miners to access deposits that are not feasible to reach through surface mining methods.

While there are surface mining techniques, such as strip mining and open-pit mining, they are typically used for deposits that are close to the surface. In contrast, underground mining is necessary for deeper deposits and offers a way to extract larger quantities of coal that would otherwise remain inaccessible. This distinction is crucial as it highlights the specific geological and environmental conditions that dictate mining practices.

The other options suggest locations that either don't apply to coal mining specifically or indicate less common scenarios. Underwater mining is not generally a practice used for coal, as coal deposits are found on land. A cavity may refer to an opening created during mining but does not represent an actual mining location. Thus, underground remains the central focus for coal extraction.