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The walk through the tunnel at Dubna looks interesting if a little damp. Is there a decent footpath? I don't think I've known a tunnel go under a canal before. There are two road tunnels in London (one east and one west) which go under the river Thames which are almost three miles long. I don't think pedestrians are allowed to walk through but it's always a little exciting to drive through. There's also some very long canal tunnels cut through hills. Back in history when canal boats were pulled by horses the horse would be walked over the hill and men lay on their backs on top of the boats and 'walked' on the ceiling of tunnel to move the boat through. We've been through several (but thankfully with a motorised boat) when we used to take canal holidays with the kids.