  The distribution of ions at the air/water interface plays a decisive role in
many natural processes. It is generally understood that polarizable ions with
low charge density are surface-active, implying they sit on top of the water
surface. Here, we revise this established hypothesis by combining
surface-specific heterodyne-detected vibrational sum-frequency generation with
neural network-assisted ab initio molecular dynamics simulations. Our results
directly demonstrate that ions in typical electrolyte solutions are, in fact,
located in a subsurface region leading to a stratification of such interfaces
into two distinctive water layers. The outermost surface is ion-depleted, and
the sub-surface layer is ion-enriched. As a result, an effective liquid/liquid
interface buried a few {\AA} inside the solution emerges, creating a second
water/electrolyte interface, in addition to the outermost air/water interface.
