noun, masculine/bjaˈduɡto/plural: viaductos
viaducto is masculine in Spanish.
viaduct; in Mexico City, a limited-access high-speed roadway below street level, built on top of a river which has been channeled underground
“Luego tenemos unos 80 kilómetros bastante llanos, en los que cruzaréis por debajo del famoso viaducto de Millau.” — Then we have about 80 fairly flat kilometers, in which you will cross under the famous Millau viaduct.