At nearly three and a half miles long, and taking sixteen years to build the Standedge Tunnel on the Huddersfield Narrow Canal is the longest, deepest, and highest canal tunnel in the country. The Standedge Tunnel is situated 645 feet above sea level, and burrows 638 feet underneath the Pennines. The final section of the tunnel was overseen by the renowned engineer Thomas Telford in 1811.
Information adapted from:
http://www.waterscape.com/in-your-area/yorkshire/places-to-go/462/standedge-tunnel
