Romans widely used arches in their architecture to provide strength and support. They built aqueducts with arches to transport water from distant sources into cities, as urban populations grew. Aqueducts consisted of stone channels and underground concrete tunnels supported by arches. Romans also incorporated arches into buildings, victory arches, and amphitheaters like the Colosseum. The keystone at the top of an arch locks the wedge-shaped voussoirs into a stable semi-circular structure that transfers weight down buttresses into the ground.