The Paraná is not an obstacle to be taken lightly. Wide, fast, and heavy with silt, it had divided Santa Fe and Entre Ríos so completely that the two provinces functioned almost as separate worlds, linked only by boats. Engineers spent decades arguing over how to bridge it. A cable-stayed span was proposed and rejected. Idea after idea collapsed under the cost and the sheer scale of the water. The breakthrough came not from a single inventor but from two politicians willing to share the risk. In 1960 the governors of the two provinces, Raúl Uranga of Entre Ríos and Carlos Sylvestre Begnis of Santa Fe, signed a joint treaty authorizing construction. The first stone was laid in 1962. The decision to build a tunnel rather than a bridge would define the project: it would be sunk, not raised.

The method was audacious. Rather than dig through rock, the builders fabricated the tunnel in pieces on land, each tube a hollow concrete cylinder 65.45 meters long, weighing 4,500 tonnes, with walls half a meter thick. One by one, the sections were floated out, lowered into a trench dredged along the riverbed, and joined end to end underwater. A German-led consortium headed by the firm Hochtief carried out the work, beginning the first tubes in 1966. When the segments were finally sealed together, they formed 2,397 meters of roadway, with access ramps reaching 270 meters at each end. Inside, the driving lane is 7.5 meters wide, the ceiling 4.41 meters overhead. It was among the earliest immersed-tube road tunnels in South America, a technique then more familiar in Europe and North America than below the Paraná.

On 13 December 1969, after nine years of treaties, dredging, and sunken concrete, the tunnel opened to traffic. What had been a ferry crossing became a five-minute drive beneath the water. The effect on the region was immediate and lasting. Until the Rosario–Victoria Bridge opened decades later far to the south, this tunnel remained the only road link between Santa Fe and Entre Ríos, and one of the few connecting two of Argentina's most productive and populous regions. By 2004, traffic counts recorded more than 2.7 million vehicle crossings in a single year, the overwhelming majority of them cars and motorcycles, the rest heavy trucks hauling freight that once had to wait for a barge.

For most of its life the tunnel carried a different name. It was originally dedicated to Hernando Arias de Saavedra, known as Hernandarias, remembered as the first governor in South America who was descended from Europeans yet born in the Americas. In 2001 the name was changed to honor the two governors whose 1960 treaty had set the whole improbable enterprise in motion. The structure is still run jointly, overseen by an interprovincial board with members from both Santa Fe and Entre Ríos. It remains a rare thing: a piece of infrastructure that does not merely cross a border but is owned in common across one, a tunnel that belongs equally to both shores it connects.