Since about 1980 Corrosion Monitoring Systems have been used in many concrete structures in aggressive environmentworldwide. While these systemswork properly in aboveground environment, some questions have arisen for submerged conditions, e.g. the outer sides of tunnels, piers in seawater or foundations in wet soil. One question concerns macro-cell formation between reinforcement in submerged concrete and in nearby aerated concrete, which might lead to severe corrosion for certain types or configurations of structures (“hollow leg”). In addition, in some cases, unexpected monitoring signals have been measured in submerged structures. The interpretation of electrochemical methods for monitoring the corrosion activity is not straightforward and new criteria have to be developed. This paper reports on an example of corrosion monitoring of an underground structure, the Green Heart Tunnel in The Netherlands. New criteria for interpretation of underground corrosion monitoring are proposed