The term phreatic (the word originates from the Greek phrear, phreat- meaning "well" or "spring") is used in hydrology and the earth sciences to refer to matters relating to groundwater (an aquifer) below the water table.
The slope of the phreatic surface is assumed to indicate the direction of groundwater movement in an unconfined aquifer.
A cave passage formed in this way is characteristically circular or oval in cross-section as limestone is dissolved on all surfaces.
Such passages form a keyhole cross-section: a round-shaped section at the top and a rectangular trench at the bottom.
They usually exhibit troglomorphism, with a loss of colour and eyesight, like the familiar blind cavefish which may also be referred to as 'phreatic fish'.