Geiger–Marsden experiment

It's how they discovered that an atom has a dense nucleus. Before that they hypothesized that atoms where like a 'plum pudding' in that electrons where evenly distributed throughout the atom like plums in a pudding. In the experiment they fired tiny particles (they can see these tiny particles because they make trails in tiny bubbles, i think that's how they see them, its something like that) at an atom and some of the particle reflected back so they did calculations and this proved the nucleus, cool!