A giant mirrored bean.
"A major feature of Millennium Park is the 110-ton elliptical sculpture designed by the celebrated British artist Anish Kapoor, one of the most prolific and respected sculptors in the world.
Cloud Gate is one of the world’s largest outdoor sculptures, at 110 tons, 66 feet long, 42 feet wide and 33 feet high. Its mirrored finish is forged of a seamless series of 168 highly polished, stainless steel “plates,” and its elliptical shape reflects the activity and lights of the park and the surrounding city skyline. With a 12-foot high concave area underneath, visitors will be able to fully experience the majestic nature of the work by literally walking through and around, as it was designed for public interaction. According to Kapoor, he wanted the piece to have a classical feel yet at the same time to be very contemporary."
http://millenniumpark.org/