The article highlights 15 secret underground cities and facilities around the world. These hidden locations serve various purposes, such as military bases, bunkers, storage facilities, and research centers. Some of the underground sites mentioned include Cheyenne Mountain Complex in the United States, Beijing Underground City in China, and The Greenbrier in West Virginia. These sites are often built to withstand natural disasters, nuclear attacks, and other threats, allowing for continued operations in times of crisis.

The article sheds light on the secrecy surrounding these underground cities and the mystery that surrounds their existence. Overall, these hidden facilities play a crucial role in protecting people and assets in times of need.