The heat of Mount Vesuvius’s volcanic eruption turned at least one victim’s brain into glass, new evidence suggests.

This discovery may be unique, researcher Pier Paolo Petrone told National Geographic. It’s rare for archaeologists to uncover brain tissue at all, he said. Due to its lack of much fibrous content, the brain is usually one of the first organs to deteriorate after death. In the rare cases when archaeologists do find preserved brain tissue, it’s usually saponified—or converted into a soapy mixture of glycerol and fatty acids. In this case, the brain matter appears to have been vitrified—liquefied at a high heat and then rapidly cooled to create a glasslike substance.

Petrone and his team aren’t sure what could have enabled the victim’s brain to be vitrified. Charred wood found near the victim’s remains, uncovered at the former site of the town of Herculaneum, indicates potential temperatures of up to 968°F (520°C). It’s unclear what may have caused the rapid cooling that turned the brain into glass, and it’s even more puzzling that brain vitrification has only been discovered in one body—so far.

“This is the first ever discovery of ancient human brain remains, vitrified by heat,” Petrone told NBC News.

[h/t Science Alert]