A Tunnel is an underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly at each end. Special tunnels, such as wildlife crossings, are built to allow wildlife to cross human-made barriers safely.

In 1818, Marc Brunel, a French engineer, invented a device that enabled workers to tunnel under rivers without having mud and water ruin their efforts. His "tunnel shield" was a rectangular cast iron wall with dozens of small shutters.

During tunnel construction, unexpected rock conditions and water penetration often present considerable challenges for engineers and miners. For this reason, tunnel construction can not be planned down to the last detail even today, despite significant advances in preliminary investigation of the rock formations.

Tunnel boring machines are generally used in hard, compact rock.

Rocks that are soft and strongly water-bearing are more challenging for the tunneling engineers and are generally excavated by drilling and blasting. The tunnels are secured continually as drilling advances and may be lined with shotcrete or concrete.