Derinkuyu is an ancient underground city in Cappadocia, Turkey, dating back to the 8th century BC. Carved out of soft volcanic rock, it served as a huge multi-level complex, capable of housing up to 20,000 people, with stables, churches, and storage rooms. The city was equipped with ingenious defensive features, including massive stone gates weighing up to 1,000 pounds, narrow passageways to bottleneck invaders, and disguised ventilation shafts to provide fresh air while remaining hidden. These traits made Derinkuyu a formidable haven in times of invasion, showcasing the advanced engineering and strategic planning of its ancient inhabitants.