One of the preserved brains found in the grave. Serrulla et al.

In 1936, between July and November, the forces of General Franco tracked down Spanish revolutionaries in northern Spain, brought them to a site outside of Burgos, executed and buried them. Teams of scientists are now working to excavate mass graves like these, and in this one, they found a rarity—45 preserved brains.

This particular mass grave was dug into “watertight clay soil with high acid content,” reports El País, and that year, the ground would have been cold and wet. The scientists believed the water flood the dead men’s skulls, keeping microbes out, and this combination of factors preserved the brains. Over time, the fatty tissue were saponified—turned into soap—and the brains shrunk, to about one-sixth of their original size. According to El País, they are now the size of about “half an apple.”

The scientists collected these preserved brains and is now keeping them in a refrigerated facility. This is now the “biggest and best preserved collection of saponified brains in the world,” reports El País. There are only about 100 cases of brains being preserved by natural causes, reports Reuters.

The team also found one preserved heart. The scientists are working to identify the men buried in this grave—104 in all—and give their families some measure of closure.