Tunnels are often curved for both safety and practicality. The most important reason is strength. A curved or round tunnel can handle the heavy pressure from the earth around it better than one with flat walls.

The forces are spread out evenly, which helps prevent cracks or collapse, just like how arches hold up bridges so well.

Curved tunnels are also useful for navigating tricky ground. Engineers often bend tunnel paths to avoid things like fault lines, unstable soil, or underground water.

A straight line isn't always the best path underground!

Modern machines that dig tunnels called TBMs, are made to create round shapes easily and efficiently.

These rounded tunnels also help with airflow, making it easier to remove smoke or fumes in train or car tunnels. Plus, in earthquake zones, their smooth shape helps absorb shaking better than boxy designs.