This is the compound from which gunpowder’s critical ingredient, potassium nitrate, is produced through a processing step.

Mammoth Cave had historically significant bat populations — still does — and the accumulation of bat guano over millennia had produced sediment with commercially exploitable concentrations of calcium nitrate in some areas, particularly the lower passages.

The War of 1812, which cut off American access to Caribbean saltpeter through a British naval blockade, created an urgent domestic demand for alternative sources.

The saltpeter operation at Mammoth Cave, which ran from around 1812 through 1815, involved enslaved workers digging cave sediment, leaching it in wooden vats of water to extract the calcium nitrate in solution, and processing the resulting solution through a chemical reaction with wood ash to produce potassium nitrate.

The wooden infrastructure of that operation — the pipes and vats and processing equipment — is partially preserved in the cave, another layer in the cave’s archaeological record.