Nothing makes travel difficult quite like Europe's tallest and most expansive mountain range. Stretching from France and Austria, the Alps extend 1,200km across eight countries in an enormous crescent formation.
Throughout history, Alpine crossings were hazardous expeditions undertaken on foot or by mule. For as long as civilisation has existed in Europe, people have endeavoured to traverse this mountain range so essential for travel, commerce and warfare.
The Romans waged battles to dominate passages through the Alps. Notable historical figures, including Hannibal — accompanied by elephants — and then later Napoleon, commanded armies across the Alps to secure a tactical military advantage.
They would have longed for something resembling the Gotthard Base Tunnel.
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The passageway linking northern and southern Europe opened in 2016. At an expense of approximately £11.5bn, it stands as the longest and deepest tunnel ever constructed.
The Gotthard tunnel is a vital travel and trade link in Europe. Utilised by freight and passenger services, it reduces the length of trips from Basel to Zurich and Milan to Lugano by up to an hour.
It spans 57km (35 miles), a 20 minute distance beneath the magnificent Alps which tower overhead. Astonishingly, its depth of 2.3km rivals some of the deepest mines on the planet.
Trains can race through at a maximum velocity of 155mph thanks to their ingenious engineering.
Whereas older tunnels wind their way up the mountainsides, the Gotthard is entirely flat. It represents the first level route through the Alps or indeed any mountain range. As the world's longest tunnel, it surpasses the Channel Tunnel stretching between England and France by 7km.
It eclipsed the previous world record holder, Japan's Seikan rail tunnel, by 3km. The Seikan tunnel links the country's two largest islands, Honshu and Hokkaido.
The tunnel was constructed to replace the original Gotthardbahn rail tunnel, an engineering feat of its era that was completed in 1882.
Plans for its successor began a century later in the 1980s. It was an ambitious and gruelling undertaking that took 17 years to complete.
With 35 miles of mountain bearing down on the passage, one of the main challenges was preventing the tunnel from caving in on itself. Fortunately, reinforced steel rings spanning the tunnel bear the enormous weight of the Alps above.
The project necessitated the use of a colossal drill measuring the length of four football pitches. At 410m long and 10m wide, it carved through 40m of mountain each day.
