Following in the footsteps of explorers Fitzroy and Darwin to Argentina’s Atlantic coast, Leon McCarron finds wildlife and wonders aplenty on the Rio Santa Cruz. But he also discovers a huge threat to this region’s biodiversity.

What was immediately apparent was how little had changed. We stood there—three people and five horses—at the estuary where saltwater meets fresh, and looked at the shape of the land in front of us.

The curve of the river, the dip of the hill, the profile of the horizon; it all perfectly matched the picture that we held in front of us. The only difference, if you were being picky, was that in the image in our hands there was a large ship that had been beached on the shingle. But we couldn’t have everything—this was more than we’d ever hoped for, and the starting point for a remarkable and poignant journey.