Mimico Creek Drain

This storm drain is relatively easily accessible from a trail the branches off Park Lawn Road just south of the Gardiner. Watch for water levels if it's rained recently. It goes on for at least a km with good ceiling height, and branches in a Y junction into two paths. The lefthand path ends after 200m or so unless you're really agile and ambitious and you intend to leap over and climb up the wall adjacent. The right-hand path continues longer and then narrows into a circular tunnel that looked too small for my comfort and interest.
No grates bar your entry, and the elevation changes a few times, and you can use the ladder/stair things on the right. Worth checking out in an area that I feel doesn't have much else worth checking out.