Hypogene caves are formed by water that has maintained its aggressiveness below the surface, independent of the surface or CO2 from the soil or other acids originating from the surface.
According to Arthur N. Palmer
Hypogene caves are formed by water from depth forced upward by hydrostatic pressure or other energy sources, independent of water supply from the surface above or nearby.
According to Alexander B. Klimchouk.
The hypogene origin of caves is a relatively recent theory introduced by Professor Klimchouk of the National Academy of Sciences of Ukraine. A distinction is made between epigenetic (originating from above) and hypogenic (originating from below). Dissolving limestone requires carbon dioxide in the water, and the weak acid dissolves the limestone. Other acids also work, but do not normally occur in nature. In classical or epigenetic karst, the water comes from rain and the carbon dioxide from the air and soil; the water is driven by gravity. In hypogeneous karst, the water comes from an aquifer, is often thermal water, and the acidity comes from a chemical reaction; the water moves by thermal convection. In many cases, the solution is not based on carbon dioxide, but on H2S or other sulfur compounds that can form in layers with various sulfides. Klimchouk suggests that hypogene karst is the second most important cave-forming process worldwide.
Two main points play a role in this process. One is the origin of the acid, and the other is the origin of the water. As always, theories, be they ever so ingenious, are tested in science and must not contradict any known facts. Besides, there are soon people who have just waited for this theory and suddenly recognize everywhere the corresponding mechanisms. Here that was among other things Arthur N. Palmer. He not only applied the theory, but also developed it further. So there are now two definitions of what hypogene caving is (see above), and both have their justification.