Mr. Nakayama exudes pride when describing the feats of his countrymen who also followed their star to the Amazon, pointing to the pioneering farming colony of Tomé-Açu. For a stretch when he was younger, Mr. Nakayama tried living in São Paulo, where far more Japanese made their home, but he felt the forest’s tug.

“The city didn’t agree with me, and I didn’t agree with the city,” he said, explaining that he viewed tropical agriculture as his calling even as his siblings prospered in urban business ventures.

Before Mr. Nakayama made his way to Airão Velho, where visitors to the ruins call him “the hermit of the jungle,” his existence was not always so lonely. He had two female companions in the past, he said. The last one, a schoolteacher, died of an unknown disease around the time he decided to make his life here. He does not have any children.

At first, Airão Velho, about several hours by boat from the city of Manaus, was not so desolate. While residents had gradually abandoned the outpost after the rubber boom, a few families had tried to repopulate the city. A schoolhouse for children from nearby communities offered some vitality to Airão Velho.

But like those eerie images of the abandoned sites near the Ukrainian city of Chernobyl, the schoolhouse now stands empty, despite scribbling still on the chalkboard and textbooks casually strewn on the floor as if pupils were simply on a yearslong field trip.

Wielding a machete, Mr. Nakayama continues to clear the foliage around the building in an effort to preserve a tidy appearance.

“I’m glad there’s someone taking care of Airão Velho,” said Victor Leonardi, a historian of Amazonia at the University of Brasília who explored the ruins here in the 1990s. “It smelled of jaguar urine back then, but it was obviously a place of riches at one point, where people dined on porcelain from England and consumed Cognac from France.”