  The butterfly effect is today commonly identified with the sensitive
dependence of deterministic chaotic systems upon initial conditions. However,
this is only one facet of the notion of unpredictability pioneered by Lorenz,
who actually predicted that multiscale fluid flows could spontaneously lose
their deterministic nature and become intrinsically random. This effect, which
is radically different from chaos, have remained out of reach for detailed
physical observations. Here, we substantiate this scenario by showing that it
is inherent to the elementary Kelvin--Helmholtz hydrodynamical instability of
an initially singular shear layer. We moreover provide evidence that the
resulting macroscopic flow displays universal statistical properties that are
triggered by, but independent of specific physical properties at micro-scales.
This spontaneous stochasticity is interpreted as an Eulerian counterpart to
Richardson's relative dispersion of Lagrangian particles, giving substance to
the intrinsic nature of randomness in turbulence.
