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The Mersey Tunnel Tours are no ordinary, sanitised heritage tour. You’ll find no between-jobs actors donning flat caps and boot polish smut on their cheeks when you volunteer to dive below the city’s streets, and explore the inner workings of city’s literal ring road by plunging into the hollow core of the George’s Dock ventilation shaft.

One of a curious cluster of soaring, windowless towers puncturing the skyline on either side of the Mersey, St George’s Dock Ventilation Shaft is a utilitarian building far lovelier than it needs to be.

In essence it’s a giant chimney stack, purifying and filtering the air pumped into the Mersey Tunnel. The Art Deco building was designed by Sir Basil Mott and J. A. Brodie with Herbert J. Rowse as architect. Egyptian motifs reflect the Tutankhamun craze that was sweeping the world in the 1920s.

Mersey Tunnel Tours

You don’t need to book onto the Mersey Tunnel Tours to spot some of the building’s finer points though. Look out for the two black basalt statues set in recesses, reflecting night and day (reflecting that the tunnel never closes), and the west facade’s seven foot high relief in Portland stone – Speed – the Modern Mercury, and, on the east facade, the black marble memorial to the workers who died in construction of the tunnel.

The base of the building houses the offices of the Mersey Tunnel’s control station – and is the embarkation point for a fascinating tour of the inner workings of the Queensway Tunnel. From here you’ll head upwards to the top of the building and a recently-abandoned control room that, perplexingly, still has one light winking on and off. Only a decade or so since it was in operation, the remaining files, notes and assorted paraphernalia of the office give it s Marie Celeste feel, stopped at a recent moment in time.

Mersey Tunnel tour

It’s all done by computers these days, of course, but for decades the Queensway Tunnel – the older and smaller of the two tunnels connecting Liverpool to Birkenhead – was run from this Victorian-meets-White Heat-meets-PCs time capsule of a room.

From here the Mersey Tunnel Tours take you on a crossing a threshold to the other side of the building. No gleaming tiles here, just bare, soot-covered brick and two of Liverpool’s biggest fans.

These huge cast-iron fans whirl and thrum, sucking mammoth lungfuls of air from the yawning cavities of the air ducts above – and it’s here, in the engine rooms, pistons pumping, you start to get a sense of scale. And of the immense engineering project which, daily, around 100,000 of us take for granted.

When work started, no tunnel of comparable diameter (44 feet/13 metres) had ever been built. Nor anything to match its length or ambition. But build it they had to do. In 1901, just 36,000 vehicles crossed the river, by ferry. By 1921, the figure had reached 640,000. The queues for the ferry gridlocked the city.

Mersey Tunnel tour

The construction Queensway tunnel was carried out by 1,700 workers and the bedrock was cut away with pick axes, by hand, with light explosives only very occasionally. The engineering feat was a resounding success – the two tunnels meeting to within an inch of true, below 80 feet of river bed, and a further 20 feet of sandstone.

Tunnelling took five years. Fitting out, several years more, with the tunnel opened – to world attention – on July 18th 1934 by King George V. Today, maintenance and new computer monitoring systems aside, surprisingly little has changed. When they built the tunnel, they built it to last.

Stand in one of the two huge fan housing chambers and there’s something of the steam punk about the super-sized, 28-feet wide, fans, their propellers large enough to cope with the exhausts of the decidedly un-green 1930s vehicles.

Mersey Tunnel tour

They’re as old as the Mersey tunnel itself – and the one we’re standing in front of has the wonderfully prescient name of The Walker Indestructible. The power of the machines sucks us ever closer. The city’s air sucked down into its innards.

The air inside the Tunnels has a pollutant concentration of only nine parts parts per million; on Liverpool’s James Street the level is 20 parts per million

Delve deeper and you find yourself below Brunswick Street, in a hollowed out cavity, a sequence of concrete arches overhead. Originally, this area was a bridge connecting the Strand to the outer wall of St George’s Dock. The arches now carry the weight of the Port of Liverpool building.

Through an opening in a crumbling brick wall, our guide throws a plumb line. It splashes into the Mersey beneath our feet. That we’re somehow in a place that’s neither river, nor land, is curiously unsettling.

You think the Three Graces are on solid ground? Think again. Like the city they represent, a river runs through them. Reach the outer perimeter of the tunnel itself, having negotiated tiny capillary-like service tunnels, interlocking doors and service ducts, you’re 120 feet down.

Mersey Tunnel tour

Air swishes past, from the echoing ventilation chamber above, and on into the receding darkness of the tunnel’s outer skin. Along the tunnel’s length it’s forced up through holes in the carriageway, forcing warmer exhaust fumes up, and out through the tunnel’s roof.

It was only when drivers in New York’s Holland Tunnel started falling asleep at the wheel that the dangers of carbon monoxide poisoning were understood. Fortunately, it was discovered in time to add the ventilation towers to the Queensway Tunnel plan.

The Mersey Tunnel Tours become a subterranean adventure that take you beneath the tunnel’s roadway to ‘Central Avenue’ an abandoned project to allow trams to shuttle beneath the Mersey. However the ferries and train companies objected, so it remained unused – and a good thing too, otherwise the tunnel would have needed to be rebuilt as new regulations relating to tunnel safety have come into effect.

Now it’s a superhighway of another dimension, carrying the fibre optic cables that keep the two sides of our region in constant communication.

At intervals along the Mersey Tunnel’s length are fire-proof chambers providing sanctuary. They’re the refuges you reach on the other side of the recently installed green ramp ways you’ll spot along the Queensway’s carriageway. Inside, they look like Sunday League changing rooms – crew benches, a toilet, bottles of water.

Outside, they’re linked to a gantry running the length of Central Avenue. Like bus-stops on a buried and long-forgotten route.

On Central Avenue – in every way a mirror image of the Queensway Tunnel’s carriageway above – is a trickle of groundwater, and silence.

• Mersey Tunnel Tours cost £6 per person and are available to people aged 10 and above. Mersey Tunnel Tours take place on selected days all year round – see website for details.

George’s Dock image: Image via