Underground space refers to areas located beneath the Earth's surface, including tunnels, caves, and subterranean facilities. This space can be used for various purposes, such as transportation, storage, and even habitation. Urban areas often utilize underground space to reduce congestion and maximize land use above ground. The development of underground space involves engineering techniques to ensure safety and stability. Subway systems, mines, and underground parking are common examples of how this space is utilized. Additionally, underground spaces can help protect against natural disasters and provide unique environments for research and recreation.