  Kondo insulators are primary candidates in the search for strongly correlated
topological quantum phases, which may host topological order,
fractionalization, and non-Abelian statistics. Within some Kondo insulators,
the hybridization gap is predicted to protect a nontrivial topological
invariant and to harbor emergent heavy Dirac fermion surface modes. We use
high-energy-resolution spectroscopic imaging in real and momentum space on the
Kondo insulator, SmB$_6$. On cooling through $T^*_{\Delta}\approx$ 35 K we
observe the opening of an insulating gap that expands to $\Delta\approx$ 10 meV
at 2 K. Within the gap, we image the formation of linearly dispersing surface
states with effective masses reaching $m^* = (410\pm20)m_e$. We thus
demonstrate existence of a strongly correlated topological Kondo insulator
phase hosting the heaviest known Dirac fermions.
