If you look at the above photo, the building on the far right with the clock on top is the Mercantile Building. Earlier this week, a duo known for scaling to the peaks of landmarks recorded themselves atop the spire.
Judging by some posts on their Twitter feeds, they appear to freeclimb, meaning they don't use harnesses. They capture each stomach-dropping moment with handheld cameras and drones. Watch the video below, and see if you can handle the view from 545 feet above the ground.