Strangelets

A strangelet—short for "strange nugget"—is a bit of "strange matter" that scientists are hunting for with particle accelerators. When strange matter encounters normal matter, it turns normal matter into strange matter. Now, normally, strange matter decays in microseconds and doesn't cause much harm. But it's theoretically possible for a chunk to stabilize. If that happens it would start turning all nearby matter into strange matter—and it wouldn't stop. In an instant the "normal" Earth would be replaced by the "strange" Earth, which scientists strongly suspect would be inhospitable to all known life forms, including us.