Marie Curie's significant discovery was that of radioactivity. She and her husband Pierre Curie conducted groundbreaking research that led to the identification of the radioactive elements polonium and radium in the late 19th century. Their work established foundational principles in the study of radioactivity, which earned Marie Curie two Nobel Prizes: one in Physics in 1903, shared with her husband and Henri Becquerel, and another in Chemistry in 1911 for her work on radium. Curie's discoveries not only advanced the field of nuclear physics but also had lasting impacts on medical treatments, particularly in cancer therapy, where radiation is utilized to target and eliminate tumors.