Cappadocia, Turkey, is famous for its fairy chimneys and hot-air balloons. But beneath the dusty valleys lies Derinkuyu, a giant underground city carved 85 meters deep. This hidden city had 18 levels and could hold up to 20,000 people. It was like a small town underground, with homes, schools, wineries, chapels, and stables. Derinkuyu is the largest excavated underground city in the world. It was used by many civilizations including the Phrygians, Persians, and Byzantine Christians. The city was an important shelter from invasions for centuries. It had narrow tunnels to confuse enemies and big stone doors to block them inside. Tiny holes in the doors allowed defenders to fight while staying safe. The city's ventilation was impressive, with more than 50 air shafts keeping fresh air flowing even when doors were closed. A deep well provided clean water and could be sealed to prevent poison. These features allowed people to live underground for months. The discovery of Derinkuyu in 1963 was unusual. A man found a hidden passage while fixing his house after his chickens disappeared into a crack. This entrance led to the city, revealing miles of tunnels and rooms below. Besides Derinkuyu, archaeologists have found more than 200 similar underground cities across Cappadocia, some connected by long tunnels. These wonders show how ancient people used the region’s soft volcanic rock to build safe shelters. Derinkuyu was finally abandoned in 1923 during the Greco-Turkish War. Today, it is a UNESCO World Heritage Site and a powerful symbol of human courage and craft. Visitors can explore its spooky corridors and imagine life hidden deep underground long ago.