It has been drawn to my attention that it’s possible to walk from London’s Bank tube station to Waterloo station without crossing any roads. How?

A couple of definitions seem necessary. By “walk” it is meant that the journey must be solely on foot and no other means of conveyance are allowed. “Crossing” a road in this case refers to directly stepping onto a road to get to the other side of it.

So how can it be done? The answer will be posted in a while after you’ve had some time to ponder.