  We designed, developed, and deployed a distributed sensor network aiming at
observing high-energy ionizing radiation, primarily gamma rays, from winter
thunderclouds and lightning in coastal areas of Japan. Starting in 2015, we
have installed, in total, more than 15 units of ground-based detector system in
Ishikawa Prefecture and Niigata Prefecture, and accumulated 551 days of
observation time in four winter seasons from late 2015 to early 2019. In this
period, our system recorded 51 gamma-ray radiation events from thundercloud and
lightning. Highlights of science results obtained from this unprecedented
amount of data include the discovery of photonuclear reaction in lightning
which produces neutrons and positrons along with gamma rays, and deeper
insights into the life cycle of a particle-acceleration and gamma-ray-emitting
region in a thundercloud. The present paper reviews objective, methodology, and
results of our experiment, with a stress on its instrumentation.
