Chances are, you've driven through numerous tunnels, but have you given much thought to the engineering behind them?
Source: The Channel Tunnel: 20 fascinating facts — telegraph.co.uk
On 18 November 1963, the first tunnel at the Dartford Crossing opened. Initially, it was estimated that two million vehicles would use the tunnel each year, but that's now risen to an annual figure of around 50 million.
The Dartford Crossing might just be part of your daily commute, but for the keen drivers among you, there are some tunnels around the world that will take your breath away. Here's a list of some of the best:
The Lærdal Tunnel in Sogn og Fjordane, Norway takes the title for the longest tunnel, spanning 15 miles across.
Construction of the tunnel began in 1995, and it opened in 2000. During this time, 2,500,000 cubic metres of rock was removed.
It uses its unique design, lights and rest areas illuminated by a lighting scheme mimicking a sunrise to keep drivers calm, alert and engaged during the 20-minute journey.
At the other end of the spectrum, Backbone Rock Tunnel in Shady Valley, Tennessee, is a little over 6 metres wide.
The tunnel came about in 1901 when the Tennessee Lumber Company wanted the shortest route possible for its lumber trains, so it blasted a big hole through the rock.
The shortest tunnel in the world is surrounded by hiking routes and picnic tables, so motorists can take a break and enjoy the view.
In the midst of the Rockies, you'll find the world's highest vehicular tunnel — the Eisenhower-Johnson Memorial Tunnel.
It has an elevation height of 3,400 metres, but being the highest tunnel isn't the only title it takes. It's also the longest mountain tunnel in the US interstate system and has the highest point on the Interstate Highway System.
For the deepest tunnel, we head back to Norway. Creating a link between the Norwegian mainland and Hareidlandet Island, the Eiksund Tunnel is part of a larger project that includes three tunnels and a bridge.
The undersea tunnel is 7,765 metres long and reaches a depth of -287 metres. Between the tunnel's deepest point and Eika Inlet, the gradient is 9.6%.
While it might not have this official title, the Guoliang Tunnel in China is certainly deemed by many to be the most dangerous.
Carved into the mountain by local villagers, it has 30 windows of various shapes and sizes, giving passengers a view. Although it's advised that drivers keep their eyes firmly on the road.
Tunnel Log in Sequoia National Park, California is perhaps one of the most unusual road tunnels you could find.
It was created after a redwood fell and blocked the road in the national park in 1937. It now serves as a great photo opportunity.
Picturesque might not be the first thought that comes to your head when you think of tunnels, but the windows in the Zion-Mount Carmel Tunnel in Zion National Park, Utah, give passengers spectacular views of the national park.