Most caves are formed by carbonic acid — slightly acidic rainwater percolating downward through limestone, dissolving it grain by grain over millennia. This is the standard model, and it explains most of the caves we know.
Lake Neuron, discovered 127 meters below the surface in Albania's Atmos Cave, was carved by the opposite mechanism. Hydrogen sulfide-rich groundwater rises from depth, oxidizes to sulfuric acid on contact with oxygenated water, and dissolves limestone from below rather than above. The cave was born from the bottom up.
The lake measures 138.3 meters long by 42 meters wide, holds 8,335 cubic meters of mineral-rich water at a steady 26°C, and represents the world's largest known underground thermal lake. Dye-tracing experiments revealed the karst system behaves like a highly connected plumbing network, with flow velocities reaching 30 kilometers per day.
Sulfuric acid speleogenesis — hypogenic cave formation — is well-documented in theory but underappreciated in practice. The standard downward-dissolution model is taught because it describes the majority of explored caves. But explored caves are biased toward accessible ones, which are biased toward those with surface openings, which are biased toward those formed by downward processes. Hypogenic caves, formed from below, may lack surface expressions entirely. Their absence from the catalog may reflect not their rarity but their invisibility.
The standard model describes the caves we can find. It may not describe the caves that exist.