  Structural imperfections such as grain boundaries (GBs) and dislocations are
ubiquitous in solids and have been of central importance in understanding
nature of polycrystals. In addition to their classical roles, advent of
topological insulators (TIs) offers a chance to realize distinct topological
states bound to them. Although dislocation inside three-dimensional TIs is one
of the prime candidates to look for, its direct detection and characterization
are challenging. Instead, in two-dimensional (2D) TIs, their creations and
measurements are easier and, moreover, topological states at the GBs or
dislocations intimately connect to their lattice symmetry. However, such roles
of crystalline symmetries of GBs in 2D TIs have not been clearly measured yet.
Here, we present the first direct evidence of a symmetry enforced Dirac type
metallic state along a GB in 1T'-MoTe$_2$, a prototypical 2D TI. Using scanning
tunneling microscope, we show a metallic state along a grain boundary with
non-symmorphic lattice symmetry and its absence along the other boundary with
symmorphic one. Our large scale atomistic simulations demonstrate hourglass
like nodal-line semimetallic in-gap states for the former while the gap-opening
for the latter, explaining our observation very well. The protected metallic
state tightly linked to its crystal symmetry demonstrated here can be used to
create stable metallic nanowire inside an insulator.
