The naturally preserved brains of 45 people have been found in a mass grave eight decades after they were shot and killed by General Francisco Franco's forces during the Spanish Civil War.

Spanish Civil War brains heart mass grave
Fernando Serrulla, a forensic anthropologist of the Aranzadi Science Society, shows one of the 45 saponified brains found in a mass grave around the area known as La PedrajaJuan Medina/Reuters

The brains from some of the bodies in the grave in an area of Burgos known as La Pedraja were preserved by very specific environmental conditions after heavy rains seeped into bullet holes in their skulls. This "saponification" process turned them into a soap-like substance.

Francisco Etxeberria, the pathologist in charge of exhuming the mass grave, told El Pais: "This is the only such case we know of. There are even two brains that still contain the bullet that killed them."

A preserved heart has also been unearthed, eight decades after it stopped beating. "An unprecedented finding," according to forensic scientist Fernando Serrulla, who worked on the dig and has published a study with details of the discovery. "Naturally preserved brains are very rare," Serrulla told Reuters. "There are only around 100 documented cases in the world. I have been working as a pathologist for 30 years and I have never seen anything like it."

Spanish Civil War brains heart
A saponified heart of a person killed in 1936 by forces of the dictator Francisco Franco which was found in a mass grave around the area known as La Pedraja, is seen at a laboratory in VerinJuan Medina/Reuters
Spanish Civil War brains heart mass grave
Fernando Serrulla, a forensic anthropologist of the Aranzadi Science Society works at a laboratory in VerinJuan Medina/Reuters
Spanish Civil War brains heart mass grave
Fernando Serrulla, a forensic anthropologist of the Aranzadi Science Society, examines one of the saponified brains of people killed by forces of the dictator Francisco FrancoJuan Medina/Reuters
Spanish Civil War brains heart mass grave
Fernando Serrulla, a forensic anthropologist of the Aranzadi Science Society, shows one of the 45 brains found in a mass graveJuan Medina/Reuters
Spanish Civil War brains heart
A mass grave which contained the remains of those killed by forces of the dictator Francisco Franco in 1936 is seen in the area known as La PedrajaJuan Medina/Reuters

The brains are being kept in a laboratory in Galicia, northwestern Spain, where Serrulla works. Brown, shrivelled to just one-sixth of their original size and with the ridges still showing, they form the largest collection of naturally preserved human brains in the world, he says. None of the preserved organs and only 16 of the 104 bodies dug up from the grave have been identified.

Rafael Martinez, the president of a socialist association killed by Franco's supporters in 1936, was recently identified as one of the bodies in La Pedraja. "If only those brains could tell of what happened there," his grandson, Miguel Angel Martinez, said.

Spanish Civil War brains heart mass grave
Miguel Angel Martinez, grandson of Rafael Martinez who is one of those were killed by forces of the dictator Francisco Franco in 1936, poses next to a mass grave in the area known as La PedrajaJuan Medina/Reuters

On 18 July 1936 a group of officers staged a military coup in Spain in an attempt to overthrow the left-wing Popular Front government. This sparked a civil war that resulted in victory for the Nationalists, who were supported by Adolf Hitler's Germany and Benito Mussolini's Italy.

It is thought there may be more than 2,000 mass burial sites across Spain dating from the 1936 to 1939 civil war and the ensuing dictatorship of General Francisco Franco. Only a handful have been dug up due to a lack of funding and Spain's "pact of forgetting" pardoning political crimes committed in the past, on its return to democracy in the 1970s. However, there is increasing interest from a new generation to face up to the country's past.

Spanish Civil War brains heart mass grave
Victoria Martinez, daughter of Rafael Martinez who was killed by forces of the dictator Francisco Franco in 1936 in the area known as La Pedraja, poses with a portrait of her father in BriviescaJuan Medina/Reuters
Spanish Civil War brains heart mass grave
A portrait of Rafael Martinez, who was killed by forces of the dictator Francisco Franco in 1936 in the area known as La Pedraja, is seen among other portraits at the family home in BriviescaJuan Medina/Reuters

Historians estimate as many as 500,000 combatants and civilians were killed on the Republican and Nationalist sides in the war. Atrocities were committed on both sides. The victors under General Franco executed thousands more after the war.