Foundations
This is what actually supports the entire bridge by carrying the load from the supporting columns, piers or towers to the ground. Since the soil and/or rock that bridges are built on top of are usually not as strong as the steel and concrete, these foundations usually need to be wide to evenly distribute the huge forces of the bridge's load.
Sometimes it is necessary to "pile" drive huge steel or concrete pins deep underground to solid rocks that can support the weight of the bridge above. This can also be done sideways when the abutments need to be anchored in place as well.
See also abutments, columns, piers and towers.