Saltpetre works

In the "Cova del Rat Penat", guano (in this case, bat excrements) deposited over thousands of years became saltpeter after being leached by the action of rainwater.

[2][3] Besides "Montepellusanus",[4] during the thirteenth century (and beyond) the only supply of saltpeter across Christian Europe (according to "De Alchimia" in 3 manuscripts of Michael Scot, 1180–1236) was "found in Spain in Aragonia in a certain mountain near the sea", (which can only be Catalonia): saraceni apellant ipsum borax et credunt quod sit alumen.

[5][4][6] In fact in 1561, Elizabeth I of England at war with Philip II of Spain, became unable to import the saltpeter (of which the Kingdom of England had no home production), and had to pay "300 pounds gold" to the German captain Gerrard Honrik for the manual "Instructions for making salpeter to growe"[7] (the secret of the "Feuerwerkbuch" -the nitraries-).

The two scientists discovered that saltpeter formed inside the walls of the caves of the doline, under certain conditions of humidity and temperature.

Following the above discovery, naturalists sent by academies from all Europe came in large number to visit Pulo di Molfetta, since the saltpeter was a fundamental ingredient in the production of gunpowder and these deposits were of considerable strategic interest.