  Since Bell's theorem, it is known that the concept of local realism fails to
explain quantum phenomena. Indeed, the violation of a Bell inequality has
become a synonym of the incompatibility of quantum theory with our classical
notion of cause and effect. As recently discovered, however, the instrumental
scenario -- a tool of central importance in causal inference -- allows for
signatures of nonclassicality that do not hinge on this paradigm. If, instead
of relying on observational data only, we can also intervene in our
experimental setup, quantum correlations can violate classical bounds on the
causal influence even in scenarios where no violation of a Bell inequality is
ever possible. That is, through interventions, we can witness the quantum
behaviour of a system that would look classical otherwise. Using a photonic
setup -- faithfully implementing the instrumental causal structure and allowing
to switch between the observational and interventional modes in a run to run
basis -- we experimentally observe this new witness of nonclassicality for the
first time. In parallel, we also test quantum bounds for the causal influence,
showing that they provide a reliable tool for quantum causal modelling.
