  Superfluid helium II contains excitations known as rotons. Their properties
have been studied experimentally for more than 70 years but their structure is
not fully understood. Feynman's 1954 description, involving rotating flow
patterns, does not fully explain later experimental data. Here we identify
volumetric, thermodynamic, colloidal, excitation, x-ray and neutron scattering
evidence that rotons are composed of interstitial helium atoms. We show in
particular that they have the same mass, effective mass and activation energy
within experimental accuracy. They readily move through the substrate, and
couple through lattice vibrations to produce quantized, loss-free flow which
corresponds to the observed superflow. Our observations revive London's 1936
conclusion that helium II has a relatively open crystal-like lattice with
enough free volume for atoms to move relative to one another, and reconcile it
with London's 1938 description of a quantum fluid.
