MEXICO CITY — At rush hour, two architects and a biologist stand on a footbridge over the highway that runs east and west through the center of this concrete cityscape. They are eager and young (the oldest, Elías Cattan, is 32) but could just as well be blind old prophets, because all they seem to see in the grime and traffic is a magnificent past and future.

Look, they say, at the odd bend in the six-lane freeway, the Viaducto Miguel Alemán; or on the side streets, at the thicket of especially large trees. Better yet, walk the median, stare down the sewer grates and glimpse the cause: the Río Piedad, or Pity River.

To most urban eyes, it is just a hidden canal of trash and feces, paved over since 1952. But to these three, among others, it is a symbol of history lost, and perhaps regained. Ignore the cough, cough of exhaust, the stink and the cost, they shout over the traffic — think bliss.

“Imagine kids singing, playing in the water and dancing,” said Delfín Montañana, 27, a biologist who works with Mr. Cattan’s firm, Taller 13 Regenerative Architecture. “This is the biggest opportunity the city has to create a real public space.”