  Defects play a key role in determining the properties of most materials and,
because they tend to be highly localized, characterizing them at the
single-defect level is particularly important. Scanning tunneling microscopy
(STM) has a history of imaging the electronic structure of individual point
defects in conductors, semiconductors, and ultrathin films, but single-defect
electronic characterization at the nanometer-scale remains an elusive goal for
intrinsic bulk insulators. Here we report the characterization and manipulation
of individual native defects in an intrinsic bulk hexagonal boron nitride (BN)
insulator via STM. Normally, this would be impossible due to the lack of a
conducting drain path for electrical current. We overcome this problem by
employing a graphene/BN heterostructure, which exploits graphene's atomically
thin nature to allow visualization of defect phenomena in the underlying bulk
BN. We observe three different defect structures that we attribute to defects
within the bulk insulating boron nitride. Using scanning tunneling spectroscopy
(STS), we obtain charge and energy-level information for these BN defect
structures. In addition to characterizing such defects, we find that it is also
possible to manipulate them through voltage pulses applied to our STM tip.
