You know that saying "we'll cross that bridge when we come to it"? There's a group in Amsterdam that has decided that's not good enough. Instead, they're planning to 3D print a steel bridge in mid-air.
3D printers are additive technologies that can create anything from guns to fashion to prosthetics for animals. These printers build something by adding on individual layers, instead of carving an object out of a chunk of wood, metal or plastic (which would be subtractive manufacturing). In this case, the engineers at the MX3D company are using steel, extruded by large industrial robots that are equipped with advanced software and welding machines.