Henri Becquerel discovered radioactivity

In late February 1896, French physicist Henri Becquerel was testing whether uranium salts, after soaking up sunlight, would emit penetrating rays like the newly discovered X-rays. Paris turned overcast, so he stowed his potassium uranyl sulfate and some photographic plates, still wrapped in black paper, inside a drawer.

When he developed the plates anyway, he found them strongly fogged. The uranium had been radiating in total darkness, with no sunlight involved. On March 1, 1896, Becquerel reported the result to colleagues in Paris: matter could spontaneously emit energetic radiation.

He had discovered a completely unknown property of matter.

The phenomenon, later named radioactivity by Marie Curie, opened a door no one had known existed. Becquerel shared the 1903 Nobel Prize in Physics with Pierre and Marie Curie, and his name now labels the SI unit of radioactivity, the becquerel, defined as one decay per second.