"Naturally preserved brains are very rare," Fernando Serrulla tells Reuters. "There are only around 100 documented cases in the world." So imagine the forensic scientist's surprise when 45 naturally preserved brains—shrunken to 20% to 30% their original size but with ridges still visible—were discovered in a mass grave in Spain. According to a study published in Science & Justice, the grave in La Pedraja contained 104 bodies of people shot and killed in 1936 during the Spanish Civil War. The mass grave is one of only a very few of the hundreds from the war that have been excavated. Only 16 of the bodies—and none of the preserved brains—from the La Pedraja grave have been identified.
There are a number of reasons behind the "exceptional finding" of the preserved brains, including the weather and manner of death. CNN reports it was colder and more rainy than normal when the victims were executed. Bullet holes in the skulls allowed rain water to seep in, but the skulls themselves kept the acidic soil away from the brains. This aided a process called "saponification," in which tissue turns soap-like. While most other body tissues decomposed, the brains were preserved. Also preserved: the heart of a single victim, which Serrulla says is unprecedented. The 45 brains—the largest collection of naturally preserved brains in the world—are being kept in a laboratory in Spain. (Read more brain stories.)