To pull this off, scientists relied on the fluorescence given off by excited oxygen atoms before being quenched by the surrounding water. This, it turns out, is an important gap in our knowledge—the oxidate properties of atomic oxygen are immensely useful in medicine and industry, so understanding how oxygen reacts in water is crucial. Previous attempts to measure atomic oxygen in water have failed since the liquid quenches any excited atoms before they can be recorded. “Measurements show that oxygen atoms persist for tens of microseconds in water, penetrating hundreds of micrometres into the liquid,” the authors wrote. Scientists measure these individual oxygen atoms by forcing atoms to absorb two photons at the same time, which sends them into an excited state.