We announce the discovery of the most massive pulsating hydrogen-atmosphere white dwarf ( WD ) ever discovered , GD 518 . Model atmosphere fits to the optical spectrum of this star show it is a 12 { , } 030 \pm 210 K WD with a \log { g } = 9.08 \pm 0.06 , which corresponds to a mass of 1.20 \pm 0.03 { M } _ { \odot } . Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase , producing an oxygen-neon-core WD . The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core . Such a massive WD should also be significantly crystallized at this temperature . The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 - 595 s and amplitudes up to 0.7 % , consistent in period and amplitude with the observed variability of typical ZZ Ceti stars , which exhibit non-radial g -mode pulsations driven by a hydrogen partial ionization zone . Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution , and can possibly place an upper limit to the mass of a carbon-oxygen-core WD , which in turn constrains Type Ia supernovae progenitor systems .