Kowloon Walled City, an enclave of murky legal status between China and British-ruled Hong Kong, filled up after the Second World War with buildings put up without architects, up to 14 storeys high and pressed against one another. By the late 1980s, about 33,000 people were packed into 2.6 hectares, more than a million per square kilometre, a record density. It was demolished in 1993-1994 and replaced by a park.
That's the estimate from a WHO-led study published in The Lancet in 2024, covering the half-century since the Expanded Programme on Immunization was launched in 1974. It works out to about six lives saved every minute, mostly infants. The measles vaccine alone accounts for around 60% of those lives.
According to the World Gold Council, about 216,000 tonnes of gold have been mined in all of human history. Because gold is so dense (19.3 tonnes per cubic metre), it would fit in a cube about 22 metres on each side, roughly the height of a seven-storey building. Strikingly, about two-thirds of it has been mined since 1950.