SETI@home has been one of the largest citizen science projects ever, with millions of users around the world.
One of the most consequential events—maybe the most consequential one throughout all of Earth's long, 4.5 billion year history—was the Great Oxygenation Event (GOE). When photosynthetic cyanobacteria arose on Earth, they released oxygen as a metabolic byproduct. During the GOE, which began around 2.3 billion years ago, free oxygen began to slowly accumulate in the atmosphere.

