Regarding the article by Provost et al. (Eos, 23 March 2004; pp. 113, 119), I have some comments concerning the interpretation of radon data and the instrumentation used for this purpose. In the first instance, it is obvious that the site presents a great number of advantages for studying the fluid transport phenomena in porous media, and I encourage the authors to continue in this investigation. My first observation is concerning the radon data shown in the paper. The authors show the data obtained in one station, using a Barasol¿ probe to measure radon in the air. The Barasol¿ probe was created initially to measure radon concentrations in the soil, and its precision is not the same as a prove like an AlphaGUARD¿, for example. Moreover, the authors are aware of this limitation. It will be interesting to compare this data with that obtained with the AlphaGUARD¿, or in the instrumented chamber described in the article that is 55 m from the tunnel entrance, named the Inner Room by the authors.