  We report the discovery of two metastable isomeric states of $^{228}$Ac at
6.28\,keV and 20.19\,keV, with lifetimes of 299$\pm$11\,ns and 115$\pm$25\,ns,
respectively. These states are produced by the $\beta$-decay of $^{228}$Ra, a
component of the $^{232}$Th decay chain, with $\beta$ Q-values of 39.52\,keV
and 25.61\,keV, respectively. Due to its low Q-value as well as the relative
abundance of $^{232}$Th and their progeny in low background experiments, these
observations potentially impact the low-energy background modeling of dark
matter search experiments.
