  The rise of topology in condensed matter physics has generated strong
interest in identifying novel quantum materials in which topological protection
is driven by electronic correlations. Samarium hexaboride is a Kondo insulator
for which it has been proposed that a band inversion between $5d$ and $4f$
bands gives rise to topologically protected surface states. However,
unambiguous proof of the existence and topological nature of these surface
states is still missing, and its low-energy electronic structure is still not
fully established. Here we present a study of samarium hexaboride by
ultra-low-temperature scanning tunneling microscopy and spectroscopy. We obtain
clear atomically resolved topographic images of the sample surface. Our
tunneling spectra reveal signatures of a hybridization gap with a size of about
$8\ \mathrm{meV}$ and with a reduction of the differential conductance inside
the gap by almost half, and surprisingly, several strong resonances below the
Fermi level. The spatial variations of the energy of the resonances point
towards a microscopic variation of the electronic states by the different
surface terminations. High-resolution tunneling spectra acquired at $100\
\mathrm{mK}$ reveal a splitting of the Kondo resonance, possibly due to the
crystal electric field.
