This paper describes air pressure, relative humidity (RH), temperature and oxygen molar fraction (xO$_2$) measurements that have been carried out over several years in a hundred meter long and 75 centimeters diameter micro-tunnel (MT) at the Andra Meuse/Haute-Marne underground research laboratory (MHM URL). Sensors were placed all along, every twenty meters inside the MT, and in the access drift. RH and xO$_2$ time evolutions exhibit very complicated profiles, showing many singularities. These features can be well explained by taking into account mainly the corrosion of the steel casings, the water evaporation from the surrounding saturated clay and a leak between the head of the MT excavation and the access drift. Airflow through the leak is driven by the drift pressure seasonal and short time fluctuations. A simple model involving coupled infinite and finite reservoirs made it possible to reproduce xO$_2$ evolution and to calculate the air fluxes associated. Drift pressure variations induce longitudinal compression-dilatation processes in the system that are closely estimated using suitable analytical formulas. By taking into account diffusion driven leaks at the casings interfaces, it was possible to build a global model from which the results compare very well with experimental data.