**Up-down instability S01-E03.***“Previously on the up-down instability. After finding out that the instability exists (S01-E01) and calculating its analytic endpoint (S01-E02), one terrifying prospect remains. What if it’s just PN? Can all of this disappear in the strong-field regime? This challenge now needs to be faced”.*

Today’s paper is the latest in our investigations of the up-down instability in binary black holes. If the primary black hole is aligned and the secondary is anti-aligned to the orbital angular momentum, the entire system is unstable to spin precession. We found this funny thing using a post-Newtonian* (*read*: approximate*)* *treatment but we couldn’t be 100% sure that this would still be true when the black holes merge and our approximation fails. So, we got our outstanding SXS friends on board and ask them if they could see the same effect with their numerical relativity (read*: the real deal!*) code. And the answer is… yes! The instability is really there! And by the way, these are among the longest numerical relativity simulations ever done.

Vijay Varma, Matthew Mould, **Davide Gerosa**, Mark A. Scheel, Lawrence E. Kidder, Harald P. Pfeiffer.

arXiv:2012.07147 [gr-qc].

Supporting material available here.