Here's something you don't see every day -- and soon may never again: a bomb shelter dug in the late-1960s and early-70s, part of an extensive but little-known marvel of human engineering called Beijing's Underground City. Many of us may have heard about the tunnel network below our feet, and we linked to an article last year about a visit to one section of the system. But last weekend I actually got to delve into the underground world myself, a particular segment that may be sealed off forever in the coming months.

In 1968, amid escalating tensions between the Soviet Union and China, Chairman Mao uttered the words, Shenwadong, chengjiliang, buchengba, a phrase that quickly became as well known as any of his Little Red Book quotations. Most commonly translated as, "Dig deep tunnels, store food and prepare for war," it became a rallying cry that mobilized approximately 300,000 Beijingers to dig, dig, dig. Using shovels, bamboo baskets and the occasional wheelbarrow over a 10-year period, citizens -- many of them children -- created a network of tunnels that extends from Tiananmen to Chongwen District and Xidan.

The 30 km of tunnels -- between eight and 18 meters underground, with an area of 85 square kilometers -- has been dubbed China's Underground Great Wall. It was designed to house nearly half the city's population in case of, say, a nuclear attack, and while each tunnel system is different, army engineers had enough foresight to install underground basketball courts, theaters and mushroom-cultivating storerooms. Many of the stones came directly from Beijing's Old City walls, which were systematically demolished in favor of this project.

The tunnels were never used for their intended purpose, as China never went to war with the U.S.S.R., and over the years they became largely forgotten and, one by one, sealed off from the public.

A few days ago I had a special opportunity to check out one of these underground tunnel systems, located off a hutong near Nanluogu Xiang. (Full disclosure: I was a source in a documentary shot by a Singaporean company called Beach House Pictures.) The manager of the shelter asked us not to divulge details on its exact location, fearing repercussions from neighbors and local authorities. Word has it that this tunnel will be sealed up sometime this summer, which means it'll find the same fate as many of the shelters around Beijing. Some have been converted into offices and storage facilities; others enjoyed a brief stint as a tourist attraction (a site in Qianmen, for example) before closing down; most will never be seen again. (Many modern apartment buildings now come equipped with "Air Defense Shelters" -- just look for green signs plastered on the sides and you'll know which ones to duck into in case of air raids.)

It was damp, chilly (18 degrees Celsius, as all the tunnels are) and, at some points, very wet. A warren of narrow hallways fed into rooms with mossy furniture, or dead ends with piles of trash, or bunkers that have never housed a soul. We found a washroom with urinals and troughs that aren't unlike what you'd find nowadays in a public bathroom on the outskirts of the city; we stumbled upon a staircase that led to a locked metal door, on the other side of which was probably a person's home.

People built all this with their hands. That was what kept going through my mind. And they had thought of everything: ventilation, trap-door escape routes, showers and wells for fresh water. It made you realize that China really does have a long history of engineering miracles, from the Great Wall to the Bird's Nest and everything in between.

Many of my relatives helped dig these tunnels. They were middle-school students back then. After the morning period of Daily Reading (of Mao's Red Book, of course), the rest of their classes would often take the form of excavating dirt, or digging out the shelter walls, or making bricks by gathering yellow dirt and molding them using wooden shoeboxes. It was tedious, labor intensive and at times painful -- my uncle described popping blood blisters until his hands could no longer get calluses; my mom said her feet bled through her socks -- but no one complained. There was no alternative. The zeitgeist of the time demanded absolute devotion to the cause, and the cause was building tunnels because Mao decreed it would be so.

Each underground segment took two to three years (or more) to complete. When it was done, kids would crawl inside to play hide-and-seek, or -- as a family friend put it -- "take a dump." To locals, it was never the relic that we might consider it today. Maybe that's why the government is so blasé about sealing it all up, content to let it disappear.

But to me, especially after learning about the human toil that went into this massive undertaking, the Underground City is a bit of living history that's as unique as Xian's terracotta warriors, as worthy of preservation as this city's hutongs and the Old Beijing gates of Deshengmen and Zhengyangmen. Hopefully I'm not guilty of wishful thinking.

Then again, as long as the Chinese live, they will build and demolish and rebuild, and rebuild again without thinking twice about what was lost. Perhaps that's the enduring lesson here -- anything is possible, even the construction of an underground world wonder that everyone seems content to never let see the light of day.