The Cygno project aims at the construction of a gaseous Time Projection Chamber (TPC) with optical readout for the high precision three-dimensional tracking of low energy nuclear and electronic recoils down to few keVs. The efficient discrimination between these two processes represents the main challenge of the modern dark matter direct detection experiments. In this context, the gaseous TPCs with optical readout are a promising and innovative technique that can reach very good energy and 3D position reconstruction capabilities thanks to the high performance of the latest generation of scientific CMOS (sCMOS) light sensors. The Cygno experimental setup is characterized by a TPC filled with a ${\rm He}$:${\rm CF_4}$ gas mixture at atmospheric pressure and equipped with a triple Gas Electron Multipliers (GEM) amplification stage. The visible light produced at the GEMs is collected by a scientific CMOS camera and by a set of fast photosensors. In this contribution we will present the 50 L prototype, called Long Imaging ModulE (LIME), foreseen to conclude the R\&D phase of the Cygno project. LIME has been recently installed underground at the Laboratori Nazionali del Gran Sasso (LNGS), with the aim of studying the performance of the Cygno experimental approach in a low background environment and developing a refined trigger and DAQ system for the future upgrades. This is a crucial step towards the development of a larger $\mathcal{O}(1 {\rm m^3})$ demonstrator, which
will be an evolution of the LIME detector.