What causes grand minima such as the Maunder Minimum?
In plain words
From about 1645 to 1715 sunspots nearly vanished, and radioactive isotopes in tree rings and ice show many such quiet spells; whether they come from random noise in the dynamo or from a deterministic switch is unknown.
Precise statement
Cosmogenic isotope records (C-14, Be-10) show a few tens of grand minima over the last $\sim 1e4\,\mathrm{yr}$, occupying of order one sixth of the time. Determine whether entry and exit arise from stochastic fluctuations in the Babcock-Leighton poloidal source (scatter of active-region tilt angles, rare anomalous active regions), from nonlinear deterministic dynamo dynamics, or from changes in meridional flow, and whether the waiting-time distribution is exponential. Answer: a model reproducing the frequency and duration distribution of grand minima and the hemispheric asymmetry of the Maunder Minimum.
What would settle it
Dynamo models calibrated on observed tilt-angle statistics whose simulated grand-minimum statistics agree with the isotope record.