England has hundreds of miles of canals. Before the railways were built everything was transported by these narrowboats. Back then the boat would have been towed by a rope attached to a horse that was lead down the towpath. When a tunnel was encountered the horse was un-hitched and lead over the top while those on the boat would lay on their back on top of the cabin with their feet on the roof of the tunnel. They would then proceed to "walk" the tunnel's roof until they reached the other end. I think the longest tunnel on the canal system is over two miles! A long way to walk on your back.