90 Th has a remarkably low-lying metastable isomer only 8.355 74 (3) eV above the ground state. The isomer's low energy produces "gamma rays" at a wavelength of 148.3821(5) nm, in the far ultraviolet, which allows for direct nuclear laser spectroscopy. Such ultra-precise spectroscopy, however, cannot begin without a sufficiently precise initial estimate of the wavelength, something that was only achieved in 2024 after two decades of effort. Combined with its convenient half-life of 1740 ± 50 s, this allows the development of a nuclear clock of unprecedented accuracy.