Atmospheric neutrino experiments measure the oscillations of neutrinos produced in the atmosphere by default assuming unitary mixing of electron-, muon-, and tau-neutrinos. The task-list for the upcoming years involves: (a) Measure the atmospheric mixing parameters with increased precision. (b) Determine the known unknowns in the framework of three neutrino flavours: ordering of neutrino masses and CP-violation. (c) Probe the default model for unknown unknowns: Is three-neutrino-mixing indeed unitary or can we see hints for additional states or interactions outside the standard model of particle physics?

In the seminar, I will give an overview of the current experimental status on these three points and the future prospects of next-generation experiments, in particular KM3NeT/ORCA. Special focus will be put on a measurement where ORCA will be already highly competitive during construction in the upcoming year(s): constraining the unitarity assumption by measuring the appearance of tau-neutrinos.