The Great Oxidation Event (GOE) or Great Oxygenation Event, also called the Oxygen Catastrophe, Oxygen Revolution, Oxygen Crisis, or Oxygen Holocaust,[2] was a time interval during the Earth's Paleoproterozoic era when the Earth's atmosphere and shallow seas first experienced a rise in the concentration of free oxygen.[3] This began approximately 2.46–2.426 billion years ago (Ga) during the Siderian period and ended around 2.06 billion years ago during the Rhyacian.[4] Geological, isotopic, and chemical evidence suggests that biologically produced molecular oxygen (dioxygen or O2) started to accumulate in the Archean prebiotic atmosphere by microbial photosynthesis. It changed the atmosphere from a weakly reducing state practically devoid of oxygen into an oxidizing one containing abundant free oxygen,[5] with oxygen levels being as high as 10% of the modern atmospheric level by the end of the GOE.[6]