Tunnel

engineering · transportation · underground

Dug or built. Tunnels are created by digging through soil, earth, or rock, or by placing them under water. They're completely covered except for openings at each end.

Secret shortcuts. Tunnels let trains, cars, and people travel under mountains, rivers, and cities instead of going over them or around them.

Breathing room. Most tunnels have special openings along their length so fresh air can get in and bad air can get out, keeping people safe inside.

Different methods. Some tunnels are dug using tunnel boring machines that slowly carve through rock, while others use immersed tube construction, which means sinking pre-made tubes under water.