Blisworth Tunnel has had a troubled history since first it first opened at the beginning of the 18th century. Water-bearing strata in the hill caused considerable stability problems and resulted in sporadic closings of the tunnel.  The tunnel lacked an inside path for tow animals, so they were led over the hill to await the emergence of the boat on the other side.  A boat was propelled through the tunnel by two men lying prone on wingboards and pushing against the side of the tunnel with their legs. "Legging" was dangerous and was discontinued in 1871 when steam tugs took over the job of propelling boats through the long tunnel.  Steam tugs, however, burned lots of fuel and made lots of smoke, which tended to concentrate in the tunnel. After a few workers died of asphyxiation, a number of ventilation shafts were dug between the tunnel and the surface.  Geological water problems continued.. In 1980 the tunnel was closed again for four years while much of the tunnel was relined with concrete.