Particle detectors of the European Space Environment Viewing and Analysis Network (SEVAN) network located on mountain peaks in Aragats (Armenia), Lomnicky Stit (Slovakia), and Musala (Bulgaria) are well suited for the detection of thunderstorm ground enhancements (TGEs). The modulation of charged particle flux by the electric field of the thundercloud results in a sizable change in the count rate of detectors, which measure fluxes of electrons, gamma rays, and high-energy muons. The relation between electric-field strength and changes of particle-flux count rates is nonlinear and depends on many unknown parameters of the atmospheric electric field and meteorological conditions. Nonetheless, employing tremendous TGEs as a manifestation of the strongest electric field in the thundercloud and by measuring fluxes of three species of secondary cosmic rays (electrons, gamma rays, and muons) by SEVAN detectors located at altitudes of approximately 3 km, we study the extreme strength of the atmospheric electric field. With the simulation of propagation of charged particles in a uniform electric field using the corsika code, we estimate the maximum potential difference in the thunderous atmosphere, which can reach approximately 500 MV.