  Following the emergence of superconducting granular aluminum (grAl) as a
material for high-impedance quantum circuits, future development hinges on a
microscopic understanding of its phase diagram, and whether the
superconductor-to-insulator transition (SIT) is driven by disorder or charging
effects. Beyond fundamental relevance, these mechanisms govern noise and
dissipation in microwave circuits. Although the enhancement of the critical
temperature, and the SIT in granular superconductors have been studied for more
than fifty years, experimental studies have so far provided incomplete
information on the microscopic phenomena. Here we present scanning tunneling
microscope measurements of the local electronic structure of superconducting
grAl. We confirm an increased superconducting gap in individual grains both
near and above the Mott resistivity $\rho_\mathrm{M} \approx 400\ \mu \Omega
cm$. Above $\rho_\mathrm{M}$ we find Coulomb charging effects, a first
indication for decoupling, and in-gap states on individual grains, which could
contribute to flux noise and dielectric loss in quantum devices. We also
observe multiple low-energy states outside the gap, which may indicate bosonic
excitations of the superconducting order parameter.
