A closer look
People have paid thousands to soak in "curative" hot springs for centuries, completely oblivious that their relaxing dip was a mild radiological event. In 1901, Ernest Rutherford and Harriet Brooks decided to measure the "emanation" coming off thorium, leading them down a rabbit hole of natural radiation. Shortly after, it was discovered that the world's most famous natural hot springs were pumping out absurd amounts of radon gas. The Earth's crust is packed with uranium and radium, which decay into radon gas that readily dissolves into groundwater. When Victorian aristocrats went to spas to "take the waters," the temporary feeling of invigoration they reported was likely just the biological stress response to bathing in heavily irradiated water.
