Twenty-five feet underground, beneath the Great Northern building lies a section of the Manchester and Salford Junction Canal. Opened in 1839, abandoned in 1922 and then converted into air raid shelters during WWII, the canal can now be explored thanks to Manchester Confidential's underground tour. It's a fantastic experience, with no noises from above reaches this far down and evidence of the war-time era still clearly visible.
For more detailed information on the canal, visit the Subterranea Britannica site. Oh, and don't be suprised if you come across a lost visitor while your down there.