The mystery has persisted for decades, attracting international attention and inspiring books and investigations by geneticists. It is one reason locals are in no hurry to try to prove their water theory. They are too busy posing for journalists and marketing their town to tourists as the “twins capital of the world.”

Image Darlene and Darles Volkweis. Most of the twins are fraternal. Credit... André Vieira for The New York Times

Some researchers have suggested the darker possibility that Josef Mengele, the Nazi physician known as the Angel of Death, was involved. Mengele, residents say, roamed this region of southern Brazil, posing as a veterinarian, in the 1960s, about the time the twins explosion began. In a book published last year, an Argentine journalist, Jorge Camarasa, suggested that Mengele conducted experiments with women here that resulted in the higher rate of twins, many of them with blond hair and light-colored eyes. The experiments, locals said, may have involved new types of drugs and preparations, or even the artificial insemination Mengele claimed to know about, regarding cows and humans.

But neither Mr. Camarasa nor any other adherent of the Mengele theory has been able to prove the escaped Nazi conducted any experiments here. Mengele, who died in Brazil in 1979, was notorious for his often deadly experiments on twins at Auschwitz, ostensibly in an effort to produce a master Aryan race for Hitler.

“People who are speculating about Mengele are doing so to sell books,” said Paulo Sauthier, a historian who runs a museum here. “He studied the twins phenomenon in Germany, not here.”

A sign at the entrance to Cândido Godói says, “Garden City and Land of Twins.” More than 80 percent of its 6,700 residents are of German descent. They began arriving around World War I, lured by the prospect of cheap land, an agreeable farming climate and incentives from the Brazilian government to colonize the area.