A viaduct is a long bridge-like structure that carries a road or railroad across a valley, gorge, or other low area. Unlike regular bridges that simply span from one side to the other, viaducts often consist of multiple arches or supports marching across the landscape like a row of giants holding up a pathway.

Picture a train track that needs to cross a wide, shallow valley. Instead of going down into the valley and back up the other side, engineers build a viaduct to keep the track level and straight. The famous Glenfinnan Viaduct in Scotland curves gracefully across a Highland glen on 21 stone arches, looking almost magical as trains pass over it.

The Romans were masters of viaduct building, constructing enormous stone viaducts to carry roads and aqueducts (water channels) across valleys and into cities. Many Roman viaducts still stand after 2,000 years. Modern viaducts might be made of steel and concrete instead of stone, but they serve the same purpose: keeping roads and railways high and level when the ground below drops away.