The kilogram was the last one -- we're out of SI base units. This was the last one not defined based on some measurable universal constant, per https://en.wikip

The kilogram was the last one -- we're out of SI base units. This was the last one not defined based on some measurable universal constant, per https://en.wikipedia.org/wiki/SI_base_unit https://en.wikipedia.org/wiki/SI_base_unit and https://en.wikipedia.org/wiki/Redefinition_of_SI_base_units https://en.wikipedia.org/wiki/Redefinition_of_SI_base_units . The meter was originally an actual physical rod (you can see the prototype on display at the fascinating French Musée des Arts et Métiers). Now it's based on the speed of light in a vacuum. The kilogram was a physical mass (and, yes, it was evaporating). Now it's finally defined in terms of the Planck constant, the second, and the meter. The second was originally 1/86400th of Earth's day. Now it's based on the radiation of cesium-133 (I don't really understand how this is measured). The ampere was originally defined based on depositing milligrams of silver from a solution of silver nitrate; it's now based on newtons and meters. The kelvin was originally defined based on water; it's still defined based on water. The mole hasn't really changed much? It's based on the number of atoms in a kilogram of carbon. The candela was originally based on a literal standard amount of light emitted by candles made from dead whales; now it's based on radiation from light sources of a specific frequency and an intensity based on the watt (which relies on kilograms, meters, and seconds).