  Recent topological band theory distinguishes electronic band insulators with
respect to various symmetries and topological invariants, most commonly, the
time reversal symmetry and the $\rm Z_2$ invariant. The interface of two
topologically distinct insulators hosts a unique class of electronic states --
the helical states, which shortcut the gapped bulk and exhibit spin-momentum
locking. The magic and so far elusive property of the helical electrons, known
as topological protection, prevents them from coherent backscattering as long
as the underlying symmetry is preserved. Here we present an experiment which
brings to light the strength of topological protection in one-dimensional
helical edge states of a $\rm Z_2$ quantum spin-Hall insulator in HgTe. At low
temperatures, we observe the dramatic impact of a tiny magnetic field, which
results in an exponential increase of the resistance accompanied by giant
mesoscopic fluctuations and a gap opening. This textbook Anderson localization
scenario emerges only upon the time-reversal symmetry breaking, bringing the
first direct evidence of the topological protection strength in helical edge
states.
