A graph is called triangle-free if it contains no copy of the triangle [= complete graph of order 3] as a subgraph.  A famous result of Grötzche (1959) asserts that every triangle-free planar graph is 3-colourable. An exceedingly short proof of this result was put forth by Thomassen (1994). At the time of writing these lines, humanity was unable to produce a humanly verifiable proof of the formidable 4-colour theorem. It is against this background that the Grötzche-Thomassen theorem should be considered.