'The prototype kilogram is stored in a vault in Sevres, France.'
The International System of Units dates back to just after the French Revolution. The kilogram was then defined as the mass of one liter of water. Officially, though, a kilogram is equal to the mass of the International Prototype Kilogram, a 130-year-old metal alloy cylinder referred to as the IPK. The IPK sits in a safe secured by three locks, inside a vault, in the basement of an International Bureau of Weights and Measures facility outside Paris.