The infamous rotten egg or sulfur smell comes from a gas called hydrogen sulfide. Hydrogen sulfide occurs naturally in water supplies because it’s produced when organic material like plants decay. Water absorbs the sulfides produced by decaying material as it passes by, picking up the smell in the process. Hydrogen sulfide in water isn’t considered dangerous, but it can encourage pipe corrosion and blacken your water. A rotten egg smell usually indicates that there’s slightly too much sulfide in your water supply.