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On 17 September 1683, Antonie van Leeuwenhoek wrote to the Royal Society describing what he had observed in scrapings from human teeth: tiny creatures, moving with apparent purpose, completely invisible to the naked eye. The Dutch draper, who ground his own lenses and built microscopes capable of magnifications up to 270 times, had become the first person in history to observe bacteria and oral microorganisms. With no university education and no institutional backing, Leeuwenhoek revealed an entire world that had existed alongside humanity for all of recorded history, yet remained utterly unknown. Also on this day: in 2022, eight cheetahs arrived at Kuno National Park in India, ending a 75-year absence of the species from the subcontinent. In 2006, a leaked audio recording brought down the Hungarian prime minister’s credibility when Ferenc Gyurcsány was heard admitting his party had lied throughout their election campaign. And that same year, Fourpeaked Mountain in Alaska erupted for the first time in at least 10,000 years, forcing geologists to reclassify what they had assumed was a dormant volcano.