Underground networks, such as metros (6 letters), are transportation systems that run beneath the surface of the earth. They are typically composed of a series of tunnels and stations, and are used to transport people and goods from one place to another. Underground networks are often used in cities, as they provide an efficient and cost-effective way to move large numbers of people quickly. They can also be used to connect different parts of a city or region, allowing for easier access to different areas.