While the blue glass windows of the John Hancock Tower make a striking impression in the Boston skyline, they were not always such a welcome sight to locals. In the building's earliest days, the structure's double-layered reflective glass panels began to detach from the building as a result of thermal stress, falling onto the sidewalk below. The fear of flying objects was so great that police blocked off neighboring streets when winds reached 45 miles per hour. At the same time, occupants of the tower's higher floors reported experiencing motion sickness as the building swayed in the wind. Fortunately for modern-day Bostonians, both architectural challenges were corrected when the window glass was replaced with single-layer tempered glass, and a harmonic absorber was installed for stability.