As the city above whirls with commotion, just below the streets and a few layers of concrete, lies an immense world of tunnels and reinforced chambers created to protect the city from allied bombings during World War II.

In the last few decades, many of these tunnels have been explored and excavated. Some of the passages sit right below public schools and other modern buildings, where the existence of the air-raid shelters has remained a mystery for decades. The discovery has allowed researchers to expand their understanding of World War II and open up a time capsule of life underground during the night raids on Milan.

One of the tunnels’ most fascinating secrets is a series of gates that could hermetically seal off chambers, protecting citizens during the use of poison gas. Many of the tunnels are also still marked with red and white arrows that direct people through the maze of passageways.

More than 20 former air raid shelters are now undergoing restoration work, in order to preserve these strange relics from World War II.