  The concept of a roton, a special kind of elementary excitation, forming a
minimum of energy at finite momentum, has been essential to understand the
properties of superfluid $^4$He. In quantum liquids, rotons arise from the
strong interparticle interactions, whose microscopic description remains
debated. In the realm of highly-controllable quantum gases, a roton mode has
been predicted to emerge due to magnetic dipole-dipole interactions despite of
their weakly-interacting character. This prospect has raised considerable
interest; yet roton modes in dipolar quantum gases have remained elusive to
observations. Here we report experimental and theoretical studies of the
momentum distribution in Bose-Einstein condensates of highly-magnetic erbium
atoms, revealing the existence of the long-sought roton mode. By quenching the
interactions, we observe the roton appearance of peaks at well-defined
momentum. The roton momentum follows the predicted geometrical scaling with the
inverse of the confinement length along the magnetisation axis. From the growth
of the roton population, we probe the roton softening of the excitation
spectrum in time and extract the corresponding imaginary roton gap. Our results
provide a further step in the quest towards supersolidity in dipolar quantum
gases.
