The mystery of the Roman concrete recipe has finally been revealed. Researchers at the University of Utah have just published a study in American Mineralogist showing how the compound of “volcanic ash, lime (calcium oxide), seawater and lumps of volcanic rock” actually did, as Pliny claimed, become stronger over time, through the very action of those waves. “Seawater that seeped through the concrete,” notes Davis, “dissolved the volcanic crystals and glasses, with aluminous tobermorite and phillipsite crystalizing in their place.” These new crystals reinforce the concrete, making it more impervious to the elements. Modern concrete, “by contrast… is not supposed to change after it hardens—meaning any reactions with the material cause damage.” (The short video above explains the process in brief.)