  The GRAPES-3 muon telescope located in Ooty, India records rapid ($\sim$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 $\geq$380 km$^2$, respectively. A charging time of 6 min to
reach 1.3 GV implied the delivery of a power of $\geq$2 GW by this thundercloud
that was moving at a speed of $\sim$60 km h$^{-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.
