It is Britain’s longest and deepest canal tunnel, snaking three-and-a-quarter miles through the heart of the Pennines.
The three-day inspection began on Monday. Engineers will spend three days inching their way through the darkness using high-powered lighting to illuminate the tunnel roof. Any cracks or deterioration will be identified and repair work scheduled.
The construction of the tunnel, which began in 1794, cost the lives of 50 men. In financial terms it cost more than £123,000 – the equivalent of £8.8 million today.
It was popular for transporting freight for more than a century but closed in 1916. An ambitious restoration saw it re-open in 2001.
With no towpath, the horse-drawn canal boats originally had to be unhitched and ‘legged’ the length of the tunnel. This required men to lie on their backs on top of the boat and ‘walk’ along the tunnel roof, moving the boat along with them. In its heyday the tunnel operated 24 hours a day and ‘legging’ took about three hours.
It has been described as “an amazing engineering achievement.”