These are the world's 12 longest tunnels

Location: London, England (from Morden to East Finchley)

Length: 17.3 miles

Completed: 1940

This portion of the Tube's Northern Line is still the United Kingdom's longest railway tunnel. Despite the exclusions of many other metro tunnels on this list, this portion makes the cut because the entire length was bored through solid rock beneath London's streets.

Location: Tailhang, China

Length: 17.3 miles

Completed: 2007

Location: Gansu, China

Length: 17.5 miles

Completed: 2016

Location: Sierra de Guadarrama, Spain

Length: 17.6 miles

Completed: 2007

Spain's Guadarrama Tunnel carries high-speed trains on the Madrid to Barcelona train line.

Location: Qinghai, China

Length: 20.3 miles

Completed: 2014

The New Guanjiao tunnel replaced an existing tunnel, built in the 1970's, through mountains in the rugged Tibetan plateau.

Location: Switzerland

Length: 21.5 miles

Completed: 2007

Lötschberg Base Tunnel was formerly the longest in the world until the Gotthard tunnel was completed nearly a decade later.

Location: Dongguan, China

Length: 24.17 miles

Completed: 2016

Location: Seoul, South Korea

Length: 31.3 miles

Completed: 2016

South Korea's Yulhyeon tunnel is virtually tied with the Channel Tunnel, and took more than three years to to carve the shortcut on the Korean peninsula.

Location: English Channel, between England and France.

Length: 31.3 miles

Completed: 1994

While the Channel Tunnel has a longer underwater length than Seikan, its total length falls about two miles short of its Japanese counterpart.

Location: Tsugaru Strait, Japan

Length: 33.5 miles

Completed: 1988

Seikan is the world's longest tunnel with an underwater segment, beating the Channel Tunnel in total length, but not in terms of how much is under water.

Location: Swiss Alps, Switzerland

Length: 35.4 miles

Completed: 2016

The Gotthard base tunnel is the longest tunnel by distance and also the deepest. Read more about the tunnels history and construction here.