Inside the Taipei 101 skyscraper in Taiwan is the world’s largest and heaviest tuned mass damper.

Essentially acting as a giant pendulum, the enormous steel sphere moves slightly back and forth to counter any motion of the building itself. It is an engineering marvel meant to limit the vibrations of the 1,667-foot tall building.

The 18-foot diameter, 660- metric ton steel sphere is suspended by eight cables in the upper stories of the tower, and is visible between the 88th and 92nd floors. The Taipei 101 Tuned Mass Damping were built and tested by A&H Custom Machine. The fabrication of the components took approximately a year to complete.

As one of the tallest structures in the world, located only 660 feet from a major fault line, earthquakes and high winds are a serious threat. In fact, visitors even caught a glimpse of the tuned mass damper in action during the 2008 Sichuan earthquake.

Many skyscrapers contain tuned mass dampers to restrain sway, and as the tops of the tallest buildings can move several feet in the wind, a tuned mass damper or similar mechanism is often necessary for maintaining structural integrity. At Taipei 101, the room-sized device is capable of moving five feet in any direction, thereby reducing sway by 30 to 40 percent.

Safety comes at a hefty price, however, as the tower’s tuned mass damper cost $4 million to build.