Attempts to convert pure water from an insulator into a metal by pressurizing the system remain in the realm of science fiction, since the estimated required pressure1 of 48 Mbar
is an order of magnitude higher than what is feasible in the laboratory nowadays and may only exist in cores of large planets or stars2-4. Indeed, recent estimates show that at
experimentally accessible pressures one can reach at best superionic water with high protonic conductivity5, but not metallic water with conductive electrons1. Here, we show that
a metallic water solution can be prepared by massive doping by electrons liberated from alkali metals. Analogous metallic solutions of liquid ammonia, which may serve in many respects
as a proxy to water, have been characterized previously6-9. However, it is a textbook knowledge that dissolution of alkali metals in water leads to an explosive chemical reaction10,11,
thus aqueous solutions with only low (sub-metallic) electron concentrations have been prepared so far12-14. We have now found a way around the explosive chemistry by adsorbing water
vapour at a pressure of about 10-4 mbar onto a train of liquid sodium-potassium alloy drops ejected from a nozzle into a vacuum chamber. This leads to a formation of a transient gold
coloured layer of water doped with ~5?1021 electrons/cm3, the metallic character of which is demonstrated by a combination of optical reflection and synchrotron x-ray photoelectron spectroscopies.