Martin Sheffield's letter and Clemente Onelli's plesiosaur expedition, the origin of Nahuelito

Martin Sheffield was an American gold prospector, hunter and otter hunter. He had settled in the Andes of Chubut and Río Negro around 1899. He presented himself as a former Texas sheriff. His friend Clemente Onelli was director of the Buenos Aires Zoo. Onelli counted 12 places between 38° and 52° S where strange animals had reportedly been seen since 1890.

The first sign was on the ground. Near "the lagoon where I have set up my hunting place," Sheffield found that "bushes and undergrowth were trodden into the ground, and so deep were the tracks that they could only have been made by some living thing of ponderous weight." The flattened grass looked as if "a very heavy cart had been dragged across it."

Then he looked at the water. He wrote that "in the middle of the lake, I saw the head of an animal. At first sight it was like some unknown species of swan, but swirls in the water made me think its body must resemble a crocodile's." He never clearly saw the body. The animal sank. Its back left a great wake.

19 January 1922. From Esquel, Sheffield wrote to Onelli. The lagoon lies at the foot of Cerro Pirque, near El Hoyo in Chubut. Today it is called the Laguna del Plesiosaurio. Onelli vouched for his friend: "I have not the least doubt that he saw a large and strange animal."

6 March 1922. Onelli made the letter public in La Nación, under a subheading asking whether it could be a plesiosaur. Onelli was ill and stayed behind. The geographer Emilio Frey led the party, with José Cinaghi, two expert shots armed with elephant rifles, local guides and a taxidermist from La Plata. Journalists Guillermo Estrella of La Nación and Augusto Vaccari of Caras y Caretas went along.

The orders were precise. Fire only in extreme need, with the heaviest calibre. Aim several shots at the neck, not the head. Set off dynamite from time to time to force the animal up. The party reached Nahuel Huapi on 30 March, then Sheffield's lagoon in April. They probed and dynamited several sites. A last boat search on the Limay River followed on 3 May. They found nothing. With winter coming on, the search was called off.

Onelli himself rejected the name. It was, he said, "a pseudonym the newspapers have given it." He suspected a surviving edentate, a ground sloth or glyptodon. C. M. Sternberg noted that plesiosaurs died out over 65 million years ago. Scientific American wrote that "Science in general utterly discredits the suggestion that a plesiosaurus…is still alive."

Contemporary naturalists settled on an otter, the very animal Sheffield hunted. Frey came to suspect a joke. Onelli privately admitted he had fallen back on the plesiosaur story to stir up interest in Patagonia. In the end, he suspected Sheffield's fertile imagination.

The plesiosaur became a craze. There were tangos, a cigarette brand and a dance step called the "Plesiosaurian Glide of Patagonia". In February 1923 the sawmill owner Primo Capraro built a wooden plesiosaur on a Ford truck chassis for the Bariloche carnival.

The legend later attached itself to Nahuel Huapi Lake. The name Nahuelito dates from 1970, when a journalist ran a supposed sighting under that headline. In 2024 a guided tour built on the legend was launched in Epuyén. In 2025 the documentary Plesiosaurios vivos revisited the story.