Environmental neutrons originate from two sources: cosmic rays and natural radioactivity. They are in equilibrium with media and are therefore sensitive to many geophysical or Sun–Earth–Moon phenomena in accordance with the source of production. A history and some results obtained with the neutron technique are overviewed and discussed. The electron–neutron detectors (en-detectors) were developed at INR RAS in the framework of the PRISMA project to study Extensive Air Shower (EAS) hadronic component through thermalized neutrons. By continuous monitoring of neutron background with the en-detectors we have found interesting variation effects in the environmental thermal neutron flux, caused by geophysical phenomena. As shown, environmental thermal neutron flux could serve as a useful instrument to study cosmic rays, geophysical phenomena and many other applications.