Investigation of an octupole breathing mode of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mi mathvariant="normal">Pb</mml:mi> <mml:mprescripts/> <mml:none/> <mml:mrow> <mml:mn>208</mml:mn> </mml:mrow> </mml:mmu

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 ${v}_{3}{4}$ data measured in experiment. Our results highlight the unique capability of capturing transient collective properties of nuclei on yoctosecond (${10}^{\ensuremath{-}24}$ s) timescales, unfeasible with low-energy nuclear reactions.