  The boson peak (BP) is a universal feature in the Raman and inelastic
scattering spectra of both disordered and crystalline materials. The current
paradigm presents the boson peak as the result of a Ioffe-Regel crossover
between ballistic (phonon) and diffusive-type excitations, where the loss of
coherence of phonons is described as a purely harmonic process due to
structural disorder. This "harmonic disorder" paradigm for the BP has never
been challenged or tested at the atomistic level. Here, through a set of
atomistically-resolved characterizations of amorphous metallic alloys, we
uncover a robust inverse proportionality between the intensity of boson peak
and the activation energy of excitations in the potential energy landscape
(PEL). Larger boson peak is linked with shallower basins and lower activation
barriers and, consequently, with strongly anharmonic sectors of the PEL.
Numerical evidence from atomistic simulations indicates that THz atomic
vibrations contributing the most to the BP in atomic glasses are strongly
anharmonic, as evidenced through very large values of the atomic- and
mode-resolved Gr\"{u}neisen parameter found for the atomic vibrations that
constitute the BP. These results provide a direct bridge between the
vibrational spectrum and the topology of the PEL in solids, and point towards a
new "giant anharmonicity" paradigm for both generic disordered materials and
for the phonon-glass problem in emerging materials for energy applications. In
this sense, disorder and anharmonicity emerge as the two sides of the same
coin.
