As we exited the tunnel on our return walk, I had a chance to talk with the Park Rangers. The pump visible in the photo was being used to empty the water out of the canal in order to do some repairs on the towpath, which is starting to fail. I asked about the different colored bricks inside the tunnel and they explained that parts of the walls have caved in over the years and been repaired with different kinds of bricks. The tunnel itself, which took from 1836-1850 to build, is holding up well.