Phreatic is a term used in hydrology to refer to aquifers, in speleology to refer to cave passages, and in volcanology to refer to a type of volcanic eruption.
https://www.kron4.com/news/national/hydrothermal-explosion-reported-at-yellowstone-causes-temporary-shutdown-usgs/
https://dailymontanan.com/2024/07/23/hydrothermal-explosion-causes-damage-in-area-of-yellowstone-national-park/
If that's small, I wonder what is considered 'big'
"Yellowstone...