The CYGNO project aims to study rare events, as low-mass (few GeV) Dark Matter (DM) particle or solar neutrino interactions, exploiting the approach of the optical readout of the scintillation light produced in the amplification in a multiple Gas Electron Multiplier (GEM) structure, of the primary ionization originated in large volume Time Projection Chamber (TPCs). The volume is filled with an He:CF4 gas mixture at atmospheric pressure. The 3D topology, and therefore direction of the recoils, is reconstructed thanks to the combined use of high-granularity, high sensitivity sCMOS cameras, for the precise tracking of the projection of the recoils on the GEM plane, and of fast light sensors in order to obtain the coordinate perpendicular to the camera plane. To conclude the R&D phase, the 50 L prototype, called Long Imaging ModulE (LIME), was moved underground at the Laboratori Nazionali del Gran Sasso (LNGS) in order to study the performance of the CYGNO experimental approach in a low background environment and to assess the contributions to the background from different sources, also comparing with Monte Carlo simulations. Stability studies and the effects of environmental condition on the light yield will be discussed, focusing on the effect of humidity on the detector response and stability amount and rate of self-sustaining micro-discharges. This is a crucial step towards the development of a large demonstrator. •A TPC filled with a gas mixture based on light atoms as target.•The combined usage of PMTs and sCMOS camera enables the 3D tracks reconstruction.•Studies on the effects of environmental condition on the light yield.