A phreatic eruption is a volcanic explosion caused by magma heating subsurface water into steam. Such an explosion can be powerful enough vaporize the landmass above it, creating a massive cloud of ash that can rapidly render a planet uninhabitable.
In the mirror universe, Emperor Philippa Georgiou devastated Qo'noS by using a hydro bomb to trigger a phreatic eruption within the planet's underground volcano system. In 2257, she attempted to do the same to Qo'noS in the prime universe. (DIS: "Will You Take My Hand?")