The Klystron Gallery at SLAC National Accelerator Laboratory, located near Menlo Park, California, is recognized as the world's longest building, measuring 3,073 meters (1.9 miles) in length. Constructed as part of the Stanford Linear Accelerator project, it serves an essential role in particle physics research, taking approximately 35 to 40 minutes to traverse on foot. Originally built in the 1960s for about $132 million, its cost today would be around £1.6 billion when adjusted for inflation. The gallery houses klystrons, powerful devices that generate microwave energy to accelerate particles to near-light speeds. The building's extraordinary length was necessary for achieving high-energy particle collisions, requiring engineers to account for the curvature of the Earth during construction. While the Klystron Gallery has long been considered the longest building, the title is debated due to other structures, such as the LIGO observatories, which are longer but classified differently. This highlights the complexities in defining what constitutes the 'longest building.'