Reaching the prolate-oblate boundary at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>116</mml:mn></mml:mrow></mml:math> via first fragmentation of a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Pt</mml:mi><mml:mprescripts/><mml:none/><mml:mn>198</mml:mn></mml:mmultiscripts></mml:math> beam: Sharp transition to triaxiality in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Ta</mml:mi><mml:mprescripts/><mml:none/><mml:mn>189</mml:mn></mml:mmultiscripts></mml:math>

How shapes of heavy nuclei evolve as their valence nucleon shells are filled continues to be a nuclear physics research frontier. The authors populated high-spin isomers in very-neutron-rich $A\ensuremath{\approx}190$ Hf-Ta-W nuclei with the pioneering fragmentation of a ${}^{198}$Pt primary beam at the NSCL. The collective level structure deduced from the subsequent $\ensuremath{\gamma}$ decay of stopped isomeric fragments using the A1900 spectrometer and GRETINA array points to a sudden transition from axially prolate shapes in the lighter Ta isotopes to a strong triaxial shape for the ${}^{189}$Ta nucleus ($Z=73$, $N=116$; 8 neutrons more than stable ${}^{181}$Ta). The results suggest that the prolate-oblate shape boundary has been reached in this previously inaccessible region of neutron-rich nuclei. They are also an important experimental benchmark for nuclear theory to describe nuclear structure and shape evolution far from stability.