I'm blown away by all the amazing responses, thanks folks!! Maybe someday I'll get up there and find the other end of the tunnel just to see it.

I know this is an old thread so I'm not sure anyone will see this, but I've always been curious about an extra subway tunnel I've seen heading southbound on the Yonge Line, going straight West. If memory serves it appears at some point south of Sheppard Yonge, sticking out perpendicularly from...