There have been several ancient underground cities discovered in Turkey over the years, so it is not clear which specific discovery is being referred to in this question. However, I can provide some general information about some of the most famous underground cities in Turkey.
One of the most well-known underground cities in Turkey is Derinkuyu, which is located in the Cappadocia region. Derinkuyu is estimated to have been built during the 8th or 7th century BCE, and it consists of several levels of tunnels, rooms, and storage areas that could accommodate thousands of people. It is believed that the city was used by early Christians as a hiding place during times of persecution.
Another notable underground city in Turkey is Kaymakli, which is also located in the Cappadocia region. Kaymakli is believed to have been built during the same time period as Derinkuyu, and it is similar in design, with several levels of interconnected tunnels and rooms. Like Derinkuyu, Kaymakli is thought to have been used by early Christians as a refuge.
Other ancient underground cities in Turkey include Özkonak, which is located near Derinkuyu and is believed to have been built around the same time; Mazi, which is located in the Kayseri province and dates back to the Hittite period; and Gaziemir, which is located in the Izmir province and is believed to have been built by the ancient Greeks.
The discovery of these underground cities is significant because they provide insight into the ancient civilizations that inhabited the region, and they highlight the remarkable engineering and architectural skills of these early peoples.