The ground uplift that occurred at Phlegrean Fields during the ''crisis'' of 1982--1984 is analyzed by taking into account the volcanological history of the area, the available geophysical information, and the data obtained through our systematic geochemical study. The chemical composition of the products of a unique historical eruption in A.D. 1538 displays a strong evolved character and fits the hypothesis of a progressive reduction in size of the magma body, which probably behaved as a closed system. The bicarbonate contents of the thermal waters in the area has increased with time, suggesting inputs of carbon dioxide into the shallow aquifers. Temperatures at the fumaroles have not changed for at least a hundred years, indicating that boiling aquifer at depth feeds the surface manifestations. Significant variations in the ratios H2S/CO2 and H2/N2 observed at the same time suggest that both the rate of ground uplifting and the increase of seismic energy release may be related to the fluctuation of a convective flow from a deep heat source. Tectonic activity can modify the permeability of deep structures and thereby trigger increased activity in surface manifestations.