Underground structures are buildings or facilities located below the surface of the ground. These can include subways, tunnels, basements, and mines. They are designed for various purposes, such as transportation, storage, or habitation, and can help save space on the surface. Constructing underground structures often involves specialized engineering techniques to ensure safety and stability. Factors like soil type, water table levels, and seismic activity are considered during the design process. Proper ventilation and lighting are also essential to create a safe and comfortable environment for occupants.