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The Forbidden City is not only a symbol of the power of the Ming and Qing dynasties, but also harbors engineering marvels far beyond its time. Even in the harsh Beijing summer, this heritage complex maintains an unusually cool temperature thanks to its three-layered underground tunnel system and 30,000 blocks of ice, a testament to the masterful ingenuity of the ancients.
Underground "ice vault": A sophisticated three-layer cold storage system.
Begun in 1420, the Forbidden City is an architectural masterpiece recognized as a UNESCO World Heritage Site in 1987. Spanning 720,000 square meters and boasting over 9,000 rooms, the complex welcomes approximately 18 million visitors annually. The Forbidden City is often associated with its dazzling glazed tile roofs and mysterious crimson walls. However, the true value that allows the palace to endure even the hottest summers at 40°C lies in its complex underground infrastructure – an ancient mechanical marvel that even modern technology cannot match.
The Forbidden City complex (Beijing, China) viewed from above.
This natural cooling system originates from the ice cellar structure designed on the principle of three concentric layers. These cellars are sturdy underground structures with brick walls several meters thick. The construction materials are meticulously selected, combining high-quality fired bricks with natural insulation layers to eliminate heat transfer from the environment. In particular, the cellar floor uses volcanic rock – a material with unique impurity filtering and heat absorption capabilities, ensuring the water quality and humidity inside the cellar remain at their ideal state.
The scale of this system's operation is truly astonishing. According to records, the imperial palace needed approximately 30,000 blocks of ice annually for cooling. The ice was meticulously prepared in mid-winter when rivers and lakes froze over. The purest blocks of ice were hand-cut, transported to underground vaults, and stacked in layers. To prevent melting, craftsmen interspersed dry straw or sawdust – inexpensive yet incredibly effective insulation materials. Thanks to its durable three-layer structure, the ice could last from winter until the end of summer without significant loss.
The real secret lies in how the ancients connected the ice cellars to the palaces above. The underground system not only stored cold air but also served as an ingenious ventilation network. Cold air from the ice cellars was channeled through underground pipes, spreading to the rooms via cleverly placed vents. This mechanism worked on the principle of convection: hot air rose and cold air from the ground filled the space. Combined with wide, sun-shading eaves and thick walls, the environment inside the Forbidden City remained comfortable despite the intense heat outside.
The ice cellar has now been renovated into a restaurant serving guests.
Utilizing natural breezes from Lake Taiyi and the surrounding moat system also plays a crucial role in temperature regulation. This is evidence of the ancient people's "harmonious with nature" architectural thinking. Instead of using machinery, they understood the laws of physics and materials to create optimal living spaces. The principles, from volcanic rock absorbing heat to air convection design, remain highly relevant today in modern green architecture and sustainable construction.
The Forbidden City is captivating not only for its splendor and court legends but also for the hidden scientific value beneath every visitor's footsteps. The underground tunnel system, with its 30,000 stone blocks, is the "cold heart" that has helped the palace endure hundreds of harsh summers. Exploring this secret reveals the masterful intellect of ancient artisans. This invaluable heritage serves as a reminder of the importance of harmoniously integrating infrastructure and environmental factors in contemporary architecture.
