At the 2020 census, the City of Manila, the heart of the Philippine capital, had about 1.85 million residents on barely 43 km²: more than 43,000 people per square kilometre, one of the highest densities of any city in the world. Paris, already among Europe's densest cities, has around 20,000. Such comparisons are tricky, though, because every country draws its city boundaries differently.
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.