The cave also contained hydrogen sulfide, a foul-smelling gas. While hydrogen sulfide can be poisonous to humans, to some types of microbes, it's a food source. Microbes in caves don't have access to other food sources like plants or algae that rely on the sun for food via photosynthesis (they're quite literally stuck where the sun don't shine). The researchers found that microbes in caves readily digest the gas when it is in the cave's atmosphere, forming sulfuric acid as a waste product. The sulfuric acid dissolves the limestone, causing the caves to grow.