Cloud Gate, Anish Kapoor’s stainless steel sculpture in Millennium Park, became one of Chicago’s defining landmarks and a major public artwork in the United States. Chosen in 1999 over a competing design by Jeff Koons, it was built from 2004 to 2006 as a seamless, mercury-inspired form made of 168 steel plates. Its mirrored exterior reflects the skyline, clouds, and visitors, while the concave chamber beneath its arch warps and multiplies those images. The project demanded complex engineering, fabrication, and structural reinforcement, and after its completion it quickly became both Kapoor’s best-known U.S. work and one of Chicago’s most popular attractions.