Underground cities are urban areas built beneath the surface of the Earth. These spaces can include residential, commercial, and recreational facilities, often designed to maximize land use and protect against harsh weather conditions. Examples include the Montreal's RESO and Wieliczka Salt Mine in Poland, which attract tourists and provide essential services. These subterranean environments often feature intricate networks of tunnels and chambers. They can serve various purposes, such as housing, shopping, and transportation. Underground cities also help reduce urban sprawl and can be more energy-efficient, making them an innovative solution for growing populations in crowded areas.