Top quarks are the only quarks that do not form classical bound states, since their large decay width prevents the formation of narrow quarkonium-like resonances. Non-relativistic QCD nevertheless predicts a broad, pseudoscalar-like enhancement of the top quark pair production rate just below the $t\bar{t}$ threshold. Two measurements by the CMS Collaboration that probe this region using the full LHC Run 2 data set of proton-proton collisions at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of $138~\mathrm{fb}^{-1}$, are briefly reviewed. The analyses use the invariant mass of the $t\bar{t}$ system and the relative top quark velocity, respectively, together with spin correlation observables, and interpret the observed excess with a simplified pseudoscalar model, $\eta_{t}$. The dilepton analysis obtains $\sigma(\eta_{t})=8.8^{+1.2}_{-1.4}$ pb and the lepton+jets analysis $\sigma(\eta_{t})=5.1\pm0.9$ pb, both differing from zero by more than five standard deviations. The two results, together with the corresponding ATLAS measurements and with recent theoretical predictions, are compared and the remaining differences are discussed.