They stayed close to the surface for the first few miles, swimming lazily with the river’s not-insignificant current. This far from the light pollution of any cities, the sky was a vaulted dome of glittering stars, so bright and so numerous it seemed as though night in this part of the world was silver and not black. It was more than bright enough to see both banks of the river and to keep the wallowing boat in the center of the channel.

Only when they neared the next village did the men dump air from their buoyancy compensators and take the RHIB down close to the bottom. Juan had taken a compass bearing before slipping below the surface, and he steered them by watching the dial’s luminous face. It was an eerie feeling, swimming in water as dark as ink. With the temperature close to his own body’s, it was as though he’d been denied all tactile sensation. They drifted for a mile, the men lazily finning to maintain steerage, before Cabrillo ordered them back to the surface.

The isolated village was well behind them, and they found they had the river to themselves. Even had there been traffic, their black gear, and the fact that only parts of their heads were exposed, would lead any native to believe the team was just a couple of branches being slowly swept down toward Argentina.

Hours rolled away. It was a faint glow emanating around the next bend that told them they were approaching the border. During their briefing, they had all seen satellite shots of the area. On the Paraguay side was a three-hundred-foot concrete quay fronting tumbledown warehouses and a customs shed. The sleepy little town was maybe four streets deep and equally wide. A white-steepled church was the tallest building. In response to the troop buildup, the local military commander had brought a detachment of soldiers to town. They were camped just north of the village in a field that ran right to the river’s red-clay bank.

The Argentine side was almost identical, except there were at least five hundred soldiers garrisoned there, and they had strengthened their position by deploying search lights on spidery towers to sweep the black river, and strung thickets of concertina wire on the dirt road connecting the two towns. The satellite pictures showed two slim boats tied to the pier near where it looked like the military was headquartered. To Juan’s eye they appeared to be Boston Whalers, and, if he had to guess, they were probably armed with machine guns and possibly grenade launchers. They would be a problem if things got hairy.

Keeping close to the bottom, but not touching it so the hull didn’t disturb the silt and leaves and kick up a telltale wake, the men swam through the formidable gauntlet. They knew they had reached the Argentine position when a shaft of light pierced the dark water. They were too deep and the river too muddy for anyone on shore to see them, but they steered away from the silvery glow anyway. On the surface, the two men in the tower watched whatever the beam revealed—empty water slowly flowing southward.