Masses and half-lives of $N = 126$ waiting-point nuclei
In plain words
The third abundance peak, near platinum and gold, forms where neutron-rich nuclei with 126 neutrons pause before decaying further. Almost none of these nuclei have been measured.
Precise statement
Measure atomic masses to 10 keV and beta-decay half-lives to 10 percent for $N = 126$ isotones with $Z = 64\text{ to }72$ and neighbors with $N = 124\text{ to }130$, and test mass models (typical rms deviation about 0.5 MeV away from stability) and half-life predictions. Answer: measured values and the resulting shift in the predicted position and width of the $A\text{ about }195$ abundance peak.
What would settle it
Multinucleon-transfer production at KISS (RIKEN) and the $N = 126$ Factory (Argonne), and fragmentation at FRIB, with Penning-trap or multi-reflection mass spectrometry and decay spectroscopy.