Most active volcanoes display eruption frequencies of 10-1000s years but a class of volcanic systems has extremely long repose-time (>10's kyr), and are deemed as extinct. Yet, some reawaken, posing a particular threat because little is known about the way they endure and stir back to life. Reawakening primarily depends on the nature of the subvolcanic magma reservoir, especially the presence and distribution of melt. Here, we integrate petrology, thermobarometry, thermomechanical models, geophysics and in situ electrical conductivity measurements to show that the magma storage beneath Ciomadul, a seemingly inactive volcano in eastern-central Europe that last erupted 30 ka, may still hold 20 to 58 km3 of water-rich silicic melt, constituting up to 20-58% in parts of the upper crustal crystal mush body. Such a melt volume exceeds the volume of erupted lava over the entire history of the volcano. This illustrates the important longevity of a magmatic reservoir at temperature above the solidus, which implies that there is still a potential for rapid mush rejuvenation. That a seemingly dead volcano like Ciomadul is actually capable of erupting in the future calls for renewed attention to “inactive” volcanoes worldwide and perhaps for a redefinition of their activity/inactivity status.