Arsenic is a chemical element that occurs naturally in rocks and soil. In its pure form, it's a gray, brittle substance, but it's usually found combined with other elements in minerals.

Arsenic is poisonous to humans and animals, which is why scientists carefully monitor water supplies in areas where it seeps into groundwater from underground rocks. Public health workers test wells and drinking water to make sure arsenic levels stay safe.

Throughout history, arsenic has appeared in surprising places: it was once used in small amounts in certain medicines and in green dyes for wallpaper and fabric. Today, chemists and environmental scientists study arsenic to understand how to remove it from water supplies and keep people safe.