Ancient concrete would seem to have little to do with volcano geology. In this presentation, I will show similarities between the caprock of the Campi Flegrei caldera (Italy), the lid of the hydrothermal system, and the Roman-era concrete for which the region was known. Both materials require a similar set of chemical reactions to give it the high durability and strength caused by microstructures of intertwining fibrous minerals. The high strength of this natural, fibrous rock is utterly fascinating. On the one hand, it helps explain the ability of the caldera to withstand periods of high-rate uplift (~2 m in two years) and delayed seismic activity, posing an interesting rock physics question of how certain rocks are capable of accommodating such deformation without immediately releasing the stored energy through rock fracturing or cracking. On the other hand, it testifies the presence of a natural process reflecting that characterizing the cementitious pastes in modern and Roman concrete. The formation of fibrous minerals by intertwining filaments confers shear and tensile strength to the caprock of the hydrothermal system, contributing to its ductility and increased resistance to fracture.