Which phenomenon most directly led to the development of quantum mechanics by challenging classical physics?

The ultraviolet catastrophe in black‑body radiation most directly forced the birth of quantum mechanics. Classical physics predicted that a cavity would emit infinite energy at short wavelengths, yet experiments showed a finite spectrum. Planck resolved this by proposing that energy is exchanged in discrete packets, or quanta. This idea broke the classical continuum and led to the full quantum theory. For example, the measured spectrum of a heated metal plate peaks in the visible range, not at the infinite blue end predicted by classical theory.