Answer: North Sea
Notes: Physicists recently highlighted Helgoland as the birthplace of quantum theory. Helgoland is a small red-sandstone island in the German Bay (Deutsche Bucht) of the North Sea. It covers less than one square kilometre and was once used as a naval fortress. In June 1925, physicist Werner Heisenberg, suffering from hay fever, moved to Helgoland for relief. There, he abandoned the idea of electrons orbiting nuclei and focused only on observable experimental data. He organized light absorption and emission data into grids called matrices.
