OVER THE MISTY MOUNTAINS

I had no idea what sort of opportunities for urban exploration China might provide, and so I went with an open mind. As it turns out, and particularly along the populous east coast, I was surprised to find somewhat limited possibilities on offer; largely due to China's rapid rate of development.

Buildings don't stay empty for long in Chinese cities, as there are always more families looking to move in... and wealthy developers with the means to make it happen.

There are construction sites aplenty but most of these are inhabited by teams of labourers, often shipped in from rural areas. During the few hours when work ceases, it's common for workers to live in crude on-site camps - rendering most construction sites near impossible to infiltrate.

It's not until you head away from the population centres that you begin to find the hidden treasures of China.

While visiting Qingdao, I started hearing rumours about forgotten German bunkers. Most of these reports were vague and inconclusive - a mysterious spy hole in a cliff face, a friend of a friend who had stumbled across a hidden tunnel entrance - but given the city's history and significance, the stories made sense.

And so it was that two of us set out from the city, hiking up the rocky slopes of Mount Fushan. The tunnel entrance we had heard about would take a few hours to reach, situated on the other side of the main peak.

Just as we began our ascent towards the aptly-named 'Dragonback Ridge' however, we were hit by a torrential storm; hard, driving rain and dangerous crosswinds, accompanied by a heavy fog.

Making our way slowly in the treacherous conditions, we were just edging around a rocky outcrop when an old WWII-style pillbox appeared out of the mist above us. We were still a long way from our target, but the apparition clearly deserved further investigation.

I scrambled up the clammy rockface, to find the turret completely sealed. It seemed to melt out of the cliff, offering only a narrow loophole - and no way inside. However, by angling a torch beam through the hole it was possible to make out a concrete shaft behind; disappearing into darkness, and the heart of the mountain.

DRAGONBACK RIDGE

It took a fair bit of searching to find an entrance. We clambered up and down narrow, slippery mountain paths, in fog so thick that it was hard to see more than a few feet ahead. Some of the rocks showed signs of blasting - shallow, unnatural indentations hollowed into the flesh of the mountain. Finally looping back around the other side of the small peak, we stumbled across a stone archway set into the inland side of the mountain.

Torches switched to full-beam we stepped out of the storm, and into a still darkness.

The tunnel beyond was mostly formed from natural rock - the bulging contours of the passage illustrating where one crater at a time had been blasted into the solid rock, joining to form a corridor. Inside, nothing stirred... other than the slow, methodical dripping of condensation from the walls. Even the raging storm outside became inaudible, as we carefully made our way deeper inside the mountain.

Branching out from this main tunnel were a number of smaller caverns and chambers; some appeared to be no more than an accidental blast in the wrong direction, while others were reinforced with solid metal walls and bulkheads.

These chambers were often marked with Chinese characters scrawled clumsily across doorframes, and would have served as storerooms, ammo dumps, dormitories. At a humidity level not far off 100%, every surface was damp to touch - and the insides of these vaulted metal chambers sparkled like electric silver where the moisture ran down over mineral deposits.

We followed the main path as it wound upwards, curving around to the right, until after a while we were able to make out faint natural light ahead.

Here the rough, rocky walls were channelled into a vaulted stone passage. We stepped tentatively through one bulkhead door and then another, to find ourselves peering up a narrow concrete shaft. A series of decaying, red-rusted rungs led up to the turret we had previously seen peering out of the mountain.

At first I had wanted to get up there for a look - but as I took hold of the nearest rung it came away from the wall in my hand, leaving a rich red stain on my palm. Climbing was clearly out of the question, so instead we headed back to where another turning had forked off from the main path - a cavernous burrow which descended, step by stone step, into the impenetrable darkness.

Ninety even steps took us down to a lower passage, where even in the darkness we could feel the clammy mist pressing in on us. Much like the natural rock passage above this basement level was lined with empty storerooms, iron doors sitting heavily on rusted hinges.

The sheer number of bolts, shutters and bars that adorned each doorway made it clear just how secure the site must have been in its heyday. The corridor terminated with a solid stone door, followed by a second and third; an impenetrable barrier, had they been sealed closed.

After the third bulkhead door we scrambled under a low stone lintel, to find ourselves back out on the mountainside. Dazed and blinking, we took our bearings... one secret underground base down, and we hadn’t even reached our destination yet.

FUSHAN WATCHTOWERS

From here it was a long trek along the top of the ridge to our next stop. We passed by a television mast guarded by a handful of wild dogs, and then down into a grassy saddle overlooking the city. By this time the storm had abated, and so we stopped for a rest beside an old look-out point - no more than a concrete hut, its interior fittings burnt, broken and scattered with litter.

The path levelled out as we followed it down the inland side of the mountain, to another tunnel entrance set high above the city skyline. The concrete archway stood partially concealed behind a crude stone shelter; a semi-circular wall set in with a viewport for sentry guns.

Passing inside, the passage rose immediately into a steep stairwell. As we climbed higher and higher inside the rocky headland we passed by a similar series of caverns and grottos hollowed out on either side. Here though, the construction was simpler - rough rock walls, and the occasional well supplying fresh spring water from deep under the mountain.

Higher up the stairs divided into three corridors, each one culminating in a lookout point. Two of these turrets faced out across the Yellow Sea, while the third offered inland views across Qingdao itself; each one was secured with a double airlock, a pair of heavy doors formed from reinforced concrete.

With these colossal blast doors closed the main network would have been securely protected against enemy missiles, mortars or grenades, which happened to find their way in through the narrow loophole of the turret.

One of these doors was partially closed as I approached, and it took all of my strength to shift the massive bulk even a few inches on its rusted hinges.

It was clear that this second tunnel network was designed purely for the purpose of defence. The three turrets between them covered a near 360-degree view of the mountain, and the caverns that sprouted from the main stair would have made convenient ammo dumps for the artillerymen above.

This installation only had one entrance, opening straight onto the mountain path. Carefully retracing our footsteps in the dark, we left the same way we came in... before stumbling across the entrance to a third tunnel network, which would prove to be the most impressive of them all.

THE HALL OF THE MOUNTAIN KING

When I spied another of the now-familiar concrete arches nearby, this time disappearing down into the ground at a 45 degree angle, it seemed at first to be a false alarm. The passage led down into an arched stone chamber beneath the earth, a square hole cut through the roof aiming a bright spotlight onto the strewn rubble beneath.

At the far side of this cave an identical corridor led back to the surface, emerging from dense green undergrowth a little further down the mountainside.

As we turned to leave however, my torchlight fell across the passage wall to reveal a small concrete hatch, hidden in the deep shadows. The heavy stone door that sealed the porthole was half covered by stones and debris, but we managed to shift enough of these aside to heave the door open, and squeeze into the small passage beyond.

The tunnel that faced us inside opened up into a corridor much like the first one we had explored; a floor of tightly packed sand and shingle, rough stone grottos blasted out on either side of the path, and the occasional iron doors leading to metal-lined vaults.

The mist here was thicker than in the other bunkers, playing havoc with my camera lens... but as we continued to explore, it soon became apparent that this section had served a more important role than either of the others.

The first sign of this was the sheer size of the network. While the other tunnels had been either a straight passage, or perhaps a couple of linked corridors, this one took a more intricate, labyrinthine form; a series of interlinking passages veering off in all directions.

Another feature that clearly distinguished this complex as a command centre, was the installation of electric lights: ceramic contants poked out of the rock walls at regularly spaced intervals, supported on rusted iron terminals.

Here the storerooms and vaults were much larger, some of them offering enough space to have served as mess halls, dormitories or kitchens. Along the walls, a series of metal pipes and vents had been installed - presumably as conduits for water, gas, or even warm air during the winter months.

Another chamber, set right in the heart of the mountain, could have served as the ideal command centre. This large U-shaped room was connected to the main thoroughfare by several reinforced steel doors, and here there appeared a much higher frequency of electrical fittings.

This main network had none of the turrets or lookout points that the other two had featured - instead it was set right in the centre of the mountain ridge, positioned between the others, and spreading much deeper into the bowels of Mount Fushan.

We tested every turning we passed: some of which culminated in dead ends, others in storerooms or metal-lined vaults. A few passages veered out and away from the central complex, emerging on distant mountain slopes.

This third network featured a total of five exit points, and each one of them was protected with the same series of three sturdy, concrete doors - rendering this mountain base impenetrable from the outside.

Other features we discovered in the caverns hinted at just how well-developed this military installation had once been. In addition to the electrical light fittings we found throughout (which were presumably installed in the later part of the 20th century, and powered by a now-absent generator), we stumbled across a series of deep wells filled with fresh mountain water, extensive metal ductwork, and a small blackened chamber set behind a hatch.

The latter, presumably, would have served as some kind of oven or boiler; at the top of the chamber a cluster of pipes and flues would have directed hot air out, before channelling it around the rest of the tunnels to provide a rudimentary central heating system.

Numerous alcoves and recesses showed traces of brick ovens, while deep in the darkness of the lowest cave, we even stumbled across a subterranean lake. Fed by natural springs, the water would once have made the perfect bathing pool - but in the many years since the tunnels were in use, had become a breeding ground for frogs and toads.

THE FUSHAN TUNNELS

It is reasonable to assume that the Fushan Tunnels have continued to evolve over an extended period of time; from the first caverns blasted into the mountain around the turn of the 20th century, through to the later installation of electric lights.

The site was designed by the Germans, and would have given them the perfect vantage point for defending Tsingtao against naval assault.

However, in 1914 the German occupation of Qingdao ended with the dramatic Siege of Tsingtao. Rather than attempting to reclaim their Chinese outpost, these colonists were instead summoned home to aid the war effort in Europe.

The Japanese Imperial fleet took control of Qingdao after this point, and it is rumoured that they extended the Fushan tunnels significantly. The faded remains of Japanese characters can still be made out, scrawled across bulkheads and lookout posts throughout the complex... alongside Chinese and even Korean script.

The city of Qingdao didn't revert back to Chinese rule until December 1922. For many years after the tunnels were allowed to fall into a state of disrepair, right up until WWII. At this point, as conflict with the Japanese was renewed afresh, the passages beneath Fushan Mountain served as artillery depots and (according to rumour) a special forces training facility. They were later used for similar purposes, during Chairman Mao's Cultural Revolution of 1966-76.

While many of the details have become lost in the mists of history, it is clear that these deep, reinforced bunkers - with their numerous gun turrets, lookout posts and secret entrances - would have served to transform the picturesque Mount Fushan into the ultimate war machine... and provided an immense tactical advantage to whoever controlled the city and port below.

The vague report which had sent us scrambling up the mountainside in a typhoon, had mentioned one possible entrance to the bunkers; my online research suggested the existence of two.

In the end we managed to find three separate tunnel networks, with a total of eight entrances between them.

It is difficult to ascertain the true extent of the Fushan tunnels; how many more such networks there might be, or how deep they spread beneath the mountains. The only certainty is that this spectacular mountain range conceals far more than meets the eye.