  Topological insulators (TI) are a phase of matter that host unusual metallic
states on their surfaces. Unlike the states that exist on the surface of
conventional materials, these so-called topological surfaces states (TSS) are
protected against disorder-related localization effects by time reversal
symmetry through strong spin-orbit coupling. By combining transport
measurements, angle-resolved photo-emission spectroscopy and scanning tunneling
microscopy, we show that there exists a critical level of disorder beyond which
the TI Bi2Se3 loses its ability to protect the metallic TSS and transitions to
a fully insulating state. The absence of the metallic surface channels dictates
that there is a change in topological character, implying that disorder can
lead to a topological phase transition even without breaking the time reversal
symmetry. This observation challenges the conventional notion of
topologically-protected surface states, and will provoke new studies as to the
fundamental nature of topological phase of matter in the presence of disorder.
