We present a novel mechanism for producing topologically stable monopoles (TSMs) from the quantum mechanical decay of metastable cosmic strings in the early universe. In an $SO(10)$ model this mechanism yields TSMs that carry two units ($4\pi/e$) of Dirac magnetic charge as well as some color magnetic charge which is screened. For a dimensionless string tension parameter $G\mu \approx 10^{ -9} - 10^{ -5}$, the monopoles are superheavy with masses of order $10^{ 15} - 10^{ 17}$ GeV. Monopoles with masses of order $10^8 - 10^{ 14}$ GeV arise from metastable strings for $G\mu$ values from $\sim 10^{ -22}$ to $10^{ -10}$. We identify the parameter space for producing these monopoles at an observable level with detectors such as IceCube and KM3NeT. For lower $G\mu$ values the ultra-relativistic monopoles should be detectable at Pierre Auger and ANITA. The stochastic gravitational wave emission arises from metastable strings with $G\mu\sim 10^{ -9}-10^{ -5}$ and should be accessible at HLVK and future detectors including the Einstein Telescope and Cosmic Explorer. An $E_6$ extension based on this framework would yield TSMs from the quantum mechanical decay of metastable strings that carry three units ($6\pi/e$) of Dirac magnetic charge.
Categories: hep-ph astro-ph.CO hep-th