Deep red garnets are found all over the world, from Thailand and Sri Lanka to the Adirondacks. They’re even the state gem of New York.

The stones that make their way into rings and necklaces must have a flawless interior. But sometimes garnets are marred with intricate traceries of microscopic tunnels. When Magnus Ivarsson, a geobiologist at the Swedish Museum of Natural History, first saw these tunnels, he wondered what could be making them.

After Dr. Ivarsson and his colleagues traveled to Thailand, they found that an assortment of evidence contradicted standard geological explanations for how the cavities might be formed. In a paper in PLOS One, the researchers are floating a new hypothesis: Perhaps what’s making the tunnels is alive.