The GRAPES-3 muon telescope located in Ooty, India records rapid (~10 min) variations in the muon intensity during major thunderstorms. Out of a total of 184 thunderstorms recorded during the interval April 2011–December 2014, the one on 1 December 2014 produced a massive potential of 1.3 GV. The electric field measured by four well-separated (up to 6 km) monitors on the ground was used to help estimate some of the properties of this thundercloud including its altitude and area that were found to be 11.4 km above mean sea level (amsl) and $\ge$380 km$^2$, respectively. A charging time of 6 min to reach 1.3 GV implied the delivery of a power of $\ge$2 GW by this thundercloud that was moving at a speed of ~60 km h$^{−1}$. This work possibly provides the first direct evidence for the generation of GV potentials in thunderclouds that could also possibly explain the production of highest energy (100 MeV) $\gamma$-rays in the terrestrial $\gamma$-ray flashes.