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Radiocarbon dating is used to determine the age of carbon-bearing material by measuring its levels of radiocarbon, the radioactive isotope carbon-14. Invented by Willard Libby in the late 1940s, it soon became a standard tool for archaeologists. Radiocarbon is constantly created in the atmosphere, when cosmic rays create free neutrons that hit nitrogen. Plants take in radiocarbon through photosynthesis, and animals eat the plants. After death, they stop exchanging carbon with the environment. Half of the radiocarbon decays every 5,730 years; the oldest dates that can be reliably estimated are around 50,000 years ago. The amount of radiocarbon in the atmosphere was reduced starting from the late 19th century by fossil fuels, which contain little radiocarbon, but nuclear weapons testing almost doubled levels by around 1965. Accelerator mass spectrometry (apparatus pictured) is the standard method used, which allows minute samples. Libby received the Nobel Prize in Chemistry in 1960. (Full article...)

The Kowloon Walled City was a densely populated enclave of China within the boundaries of Kowloon City in Hong Kong. Built as an Imperial Chinese military fort, the walled city became a de jure enclave after the New Territories were leased to the United Kingdom in 1898. Its population increased dramatically after the end of the Japanese occupation of Hong Kong during World War II, attracting mostly refugees fleeing the renewed Chinese Civil War. The walled city was crossed by alleyways, often only 1 to 2 metres (3.3 to 6.6 ft) wide, with poor lighting and drainage. Construction went unregulated, and most of the roughly 350 buildings were built with poor foundations and few or no utilities. The population was estimated at 33,000 in 1987, giving the walled city a density of around 1,255,000 per square kilometre (3,250,000/sq mi), making it the densest location in the world at the time. The walled city was demolished between 1993 and 1994 and the site is now occupied by Kowloon Walled City Park.

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Accelerator mass spectrometer used for radiocarbon dating

Hanna Cavinder

Jair Bolsonaro

Richard III of England