  The most common species in liquid water, next to neutral H$_2$O molecules,
are the H$_3$O$^+$ and OH$^-$ ions. In a dynamic picture, their exact
concentrations depend on the time scale at which these are probed. Here, using
a spectral-weight analysis, we experimentally resolve the fingerprints of the
elusive fluctuations-born short-living H$_3$O$^+$, DH$_2$O$^+$, HD$_2$O$^+$,
and D$_3$O$^+$ ions in the IR spectra of light (H$_2$O), heavy (D$_2$O), and
semi-heavy (HDO) water. We find that short-living ions, with concentrations
reaching $\sim 2\%$ of the content of water molecules, coexist with long-living
pH-active ions on the picosecond timescale, thus making liquid water an
effective ionic liquid in femtochemistry.
