Marie Curie studied physics and mathematics in Paris and began researching radioactive substances after the discovery of X-rays and uranium's radiation by Henri Becquerel. Through meticulous experiments, she found that uranium and thorium were radioactive and hypothesized the existence of unknown elements with similar properties, ultimately leading to the isolation of polonium and radium.

“To their immense surprise the radioactivity of pitchblende far exceeded the combined radioactivity of the uranium and thorium contained in it.”

Encyclopedia of World Biography Online, Gale