A term used to describe speleogenesis that occurs at or below the level of the water table. Passages formed phreatically are typically horizontal passages, rounded and smoothed by the passage of underground water. "Fossil trunks" are always formed phreatically, at least initially.
The term phreatic is used in geology to refer to matters relating to underground water below the water table (the word originates from the Greek phrear, phreat- meaning "well" or "spring"). "Phreatic surface" is a synonym for "water table."
Phreatic explosions are caused by ground water being heated by rising magma to high temperatures. The phase change from superheated liquid to vapour close to the earth's surface causes explosive activity.