Thousands of cubic kilometers of magma lie in the upper crust below supervolcanoes such as Yellowstone (USA), Toba (Indonesia), and Taupo (New Zealand). Most of these systems are identified because of surface geomorphology and eruptive deposits. Recognizing such volcanoes without surface evidence is challenging, causing large magmatic reservoirs to go unnoticed. The Tuscan Magmatic Province, Italy, features only sparse Quaternary volcanic activity, but subsurface data indicate the presence of supercritical fluids at shallow depths. Here we show that more than 5'000 km³ of magma and partial melt are stored in the middle crust of the Tuscan Magmatic Province, Italy. This fuels the high-enthalpy geothermal systems of the region. Such volumes are comparable to those of mid-crust reservoirs beneath recognized supervolcanoes. The discovery of large volumes of magma is critical to explain the long-term evolution of mature magmatic systems and to understand the behavior of large magmatic provinces.