Relativistic heavy ion collisions provide a unique opportunity to probe the nuclear structure by taking an instantaneous snapshot of the colliding nuclei and converting it into momentum anisotropies of final emitted hadrons. A long-standing puzzle of too large a ratio of the elliptical-to-triangular ($v_2$-to-$v_3$) anisotropies in ultracentral ${}^{208}\mathrm{Pb}+{}^{208}\mathrm{Pb}$ collisions at the Large Hadron Collider (LHC) cannot be solved simply by hydrodynamic simulations with initial conditions containing the spherical or certain deformed shape of ${}^{208}\mathrm{Pb}$. In this Letter, using the iEBE-VISHNU relativistic viscous hydrodynamic hybrid model simulations with the ${\mathrm{T}}_{\mathrm{R}}\mathrm{ENTo}$ initial condition, we show that a dynamic octupole deformation—a shape-breathing mode of ${}^{208}\mathrm{Pb}$—could potentially solve the $v_2$-to-$v_3$ puzzle and simultaneously describe the $v_3\left\{4\right\}$ data measured in experiment. Our results highlight the unique capability of capturing transient collective properties of nuclei on yoctosecond (${10}^{-24}$ s) timescales, unfeasible with low-energy nuclear reactions.

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• Collective flow
• Nuclear shapes and moments
• Quark-gluon plasma
• Relativistic heavy-ion collisions