Scientific knowledge of the geologic history of Vesuvius comes from core samples taken from a plus bore hole on the flanks of the volcano, extending into Mesozoic rock. Cores were dated by potassium-argon and argon-argon dating. The area has been subject to volcanic activity for at least 400,000 years; the lowest layer of eruption material from the Somma caldera lies on top of the 40,000 year‑old Campanian ignimbrite produced by the Campi Flegrei complex.