Pozzolana was a fine sandy volcanic ash, originally discovered and dug at Pozzuoli in the region around Vesuvius, but later at a number of other sites. Vitruvius speaks of four types of pozzolana. It is found in all the volcanic areas of Italy, in various colours: black, white, grey and red. With lime, it acts like Portland cement, and will set under water. It transformed the possibilities for making concrete structures, although it took the Romans some time to discover its full potential.