Penetrating seawater carries bacteria that form colonies – biofilm. The bacteria attack the concrete that is supposed to prevent stones from loosening from the tunnel walls. The concrete becomes porous and risks breaking apart. The biofilm gets its distinctive colour from rusting reinforcement in the shotcrete.

image credit: Chalmers University of Technology I Britt-Marie Wilén

released with: https://news.cision.com/chalmers/r/concrete-in-road-tunnels-decomposes-unexpectedly-fast,c4135767