The nuclear two-photon or double-gamma (2γ) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. This rare nuclear process has first been measured at the IKP TU Darmstadt (C. Walz, H. Scheit, N. Pietralla, T. Aumann, R. Lefol, V.Yu. Ponomarev, Nature (London) 526 (2015) 406) to occur also in competition to allowed one-step decays. The 2γ decay rate has now been observed (D. Freire-Fernandez et al., Measurement of the Isolated Nuclear Two-Photon Decay in 72Ge, Phys. Rev. Lett. 133 (2024) 022502) for the first time in ion beams stored in an ion-storage ring at GSI. The newly developed technique can be applied to isomers with excitation energies down to ∼100 keV and half-lives as short as ∼10 ms. The half-life for the 2γ decay of the first-excited 0⁺ state of bare ⁷²Ge ions was determined to be 23.9(6) ms, which strongly deviates from previous expectations.