  The mediators of neutrino mass generation can provide a probe of neutrino
properties at the next round of high-energy hadron (FCC-hh) and lepton
colliders (FCC-ee/ILC/CEPC/CLIC). We show how the decays of the Higgs triplet
scalars mediating the simplest seesaw mechanism can shed light on the neutrino
mass scale and mass-ordering, as well as the atmospheric octant. Four-lepton
signatures at the high-energy frontier may provide the discovery-site for
charged lepton flavour non-conservation in nature, rather than low-energy
intensity frontier experiments.
