  Turbulence generally arises in shear flows if velocities and hence inertial
forces are sufficiently large. In striking contrast, viscoelastic fluids can
exhibit disordered motion even at vanishing inertia. Intermediate between these
cases, a novel state of chaotic motion, `elasto-inertial turbulence' (EIT), has
been observed in a narrow Reynolds number interval. We here determine the
origin of EIT in experiments and show that characteristic EIT structures can be
detected across an unexpectedly wide range of parameters. Close to onset a
pattern of chevron shaped streaks emerges in excellent agreement with linear
theory. However, the instability can be traced to far lower Reynolds numbers
than permitted by theory. For increasing inertia, a secondary instability gives
rise to a wall mode composed of inclined near wall streaks and shear layers.
This mode persists to what is known as the `maximum drag reduction limit' and
overall EIT is found to dominate viscoelastic flows across more than three
orders of magnitude in Reynolds number.
