Toronto's underground labyrinth
Businesses built pedestrian tunnels "so that their employees could flow in smoothly, avoiding the congested streets (and, in winter, the cold). Shops quickly started to be added. After a few businesses had done this, a ‘network effect’ emerged whereby other businesses started to add their own tunnels to the system, benefiting from the existing tunnels while also making them more useful."
https://www.worksinprogress.news/p/torontos-underground-labyrinth?ref=ben