  Last year, iron was reported to become superconducting at temperatures below
2K and pressures between 15 and 30 GPa. The evidence presented was a weak
resistivity drop, suppressed by a magnetic field above 0.2 T, and a small
Meissner signal. However, a compelling demonstration, such as the occurrence of
zero resistance, was lacking. Here we report the measurement of a complete
resistive transition at 22.2 GPa with an onset slightly above 2 K in two very
pure samples of iron, of different origins. The superconductivity appears
unusually sensitive to disorder, developing only when the electronic mean free
path is above a threshold value, while the normal state resistivity is
characteristic of a nearly ferromagnetic metal.
