Legend had it that a county named Eternal Clarity (永清县) located halfway between Beijing and Tianjin was once a bloody battlefield a thousand years ago. The conflicts lasted for about 200 years between the Northern Song Dynasty and nomadic Tartars from the north who kept attacking the Middle Kingdom. It is said that of 55 villages in the area, 36 reflect this piece of history in their names, from Tartar’s barracks (鞑子营) to Marshal Han’s Alter (韩台).

The Song was the most intellectually oriented and technologically advanced Dynasty in Chinese, and probably, in world history, but militarily weak, while Tartars are known for being a potent war machine. Then how on earth these two peoples a world apart in terms of martial strength could be locked in a stalemate for two centuries?

The secret lies in the Great Walls, as it is now believed. This is not only the Great Wall standing on the ground that the world all know but also the Great Wall hidden underground that even the Chinese didn’t realize, until very recently.

To call it a Great Wall is not quite precise, as it is more a tunnel than a wall; in fact, it is not A tunnel, but a complicated spatial matrix covering 1,600 square kilometers of area that forms an invisible world under.

It might be hidden forever from the world’s view if on a particular day in 1951 the floor in a farmhouse in the area did not suddenly collapse, revealing a secret world beneath. The initial exploration by the locals discovered a hall with approximately 150 sqms of floor space directly under the farmhouse. In the hall there is a huge kang (bed stove), on the kang there are lamps, in the lamps there are candles, and some of the candles are half-burnt. Around the hall, there are dozens of doors, and behind each door is a winding passageway linking to more doors and more halls.

But it had to wait until 37 years later in 1988, an official investigation took place and the magnificent shadow world was rediscovered.

Chinese military historians regard this invisible Great Wall as having equal importance in defending China from outside attacks as the visible Great Wall on the ground.

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