A culvert is a tunnel that carries water under a road, railroad, trail, or embankment. Picture a stream flowing through the countryside: when workers build a road across that stream, they can't just pile dirt over the water and hope for the best. Instead, they install a culvert, which is typically a large pipe or tunnel made of metal, concrete, or plastic that lets the water keep flowing underneath while cars drive safely overhead.

Culverts come in many sizes. Some are small tubes barely wide enough for a trickle of water, while others are massive tunnels large enough to walk through. Drive along any country road after a rainstorm and you'll see water flowing out of culverts beneath the pavement. Without them, roads would flood and wash away, and natural streams would be blocked.

The word can also refer to any similar tunnel-like drain. Storm drains in cities often use culverts to channel rainwater away from streets and into sewers or nearby waterways. Engineers must calculate the right size for each culvert: too small, and water backs up and floods the road; too large, and it's wastefully expensive to build.