For the vast majority of the world which uses the metric system, this is a cornerstone.
.....the actual International Prototype Kilogram, or IPK, created in 1879 as the official standard of mass.
It's not something you can causally stop by to see in a display window.
It is kept under a triple bell jar inside a temperature- and humidity-controlled vault in a secure room within the Parc de Saint-Cloud enclave of the International Bureau of Weights and Measures, or BIPM.
Over the course of its century-plus lifetime, the IPK has emerged only three times to serve “campaigns” of active duty, most recently in 1988–1992, when it participated in a formal verification of all kilogram prototypes belonging to the 51 Meter Convention member states.
Even so, a problem occurred.
On that occasion, however, the IPK itself was found wanting. Despite all the protective protocols and delicate procedures, it had mysteriously changed. No one can say whether the IPK has lost weight (perhaps by the gradual escape of gases trapped inside it from the start) or if most of the prototypes have gained (possibly by accumulating atmospheric contaminants). The difference is approximately 30 micrograms —30 billionths of a kilogram—in a hundred years.
This messes with a lot of things.
In fact, more than mass hangs in the balance, for the kilogram is tied to three other base units of the International System of Units (SI), namely the ampere, the mole, and the candela. Several more quantities—including density, force, and pressure—are in turn derived from the kilogram.
This has led to the suggestion that it be replaced.
One invariant vying to replace the IPK is Planck’s constant, which could be determined via an experimental device called a watt balance. Alternatively, researchers may successfully express mass in terms of Avogadro’s number (which is tied to the unchanging mass of individual atoms), provided they can count the atoms in a crystal of silicon-28.
Not nearly as elegant as a hunk of metal, but reproducible. Whatever happens, it won't be soon.
But neither of these complex, costly endeavors is likely to yield a new standard in time for the next meeting of the General Conference of Weights and Measures, scheduled for 2011.