It was Henry Becquerel who discovered radioactivity, quite by accident in 1896, and this triggered a lot of research in this field.
May radioactive elements were discovered and in 1899, Ernest Rutherford discovered that uranium compounds produce three different kind of radiation.
Mysterious from the start, radio-activity attracted researchers who struggled to understand it.
The work of Marie and pierre curie, that dealt with changes in the atomic nucleus, also led the way toward modern understanding of the atom as an entity that can be split to release enormous energy. These efforts, in time, caused the birth of nuclear science.