Arnd Wiegmann/Reuters
For centuries, mountains and lakes posed massive problems for engineers.
In both cases, they make travel nearly impossible and can make trips longer by days or months to bypass obstacles.
However, with the invention of Marc Isambard Brunel and Thomas Cochrane's invention of the tunneling shield in the 19th century, that all began to change. Tunnels could go deeper - underwater through bedrock - allowing for quicker travel times.
From particle accelerators to portals through Switzerland's famous Alps, today's tunnels built with advanced Tunnel Boring Machines make Brunel's original London Thames tunnel seem tiny by comparison. For the purposes of our roundup, we excluded most metro tunnels, which are often built by cut-and-cover methods instead of boring, and water tunnels, which humans can't travel through (easily, at least.)
These are the world's 12 longest tunnels: