A two-mile-long structure in California, widely regarded as the world's longest building, is so vast that the curvature of the Earth must be considered when measuring it. The Klystron Gallery at SLAC National Accelerator Laboratory near Menlo Park extends for 3,073 metres (1.9 miles) and takes 35 to 40 minutes to walk from end to end at an average pace. Its size is so great it can be seen from space.
Built as part of the Stanford Linear Accelerator project, the gallery houses rows of machines called klystrons, powerful microwave amplifiers that generate radiofrequency energy to accelerate electrons to near light speed inside a tunnel below. Scientists compare them to kitchen microwave technology, but these machines are about 60,000 times more powerful.
The original SLAC project cost approximately $114 million to build in the 1960s, with an additional $18 million spent on research and development, totalling roughly $132 million (£98 million). Adjusted for inflation using the Bank of England's calculator, that figure amounts to £1.6 billion in today's money.
The underground accelerator beneath the gallery is one of the straightest engineered objects ever constructed. Surveyors had to account for the Earth's curvature when aligning the system to ensure the beam remained precise across almost two miles.
SLAC opened in 1962 and is operated by Stanford University on behalf of the U.S. Department of Energy. It remains a leading centre for particle physics, X-ray science, astrophysics, and other fields, contributing to multiple Nobel Prize-winning discoveries. Parts of the original accelerator now power the Linac Coherent Light Source, one of the most powerful X-ray lasers ever built.
The title of world's longest building may be disputed since 1999, when the twin facilities of the Laser Interferometer Gravitational-Wave Observatory (LIGO) were completed. Each LIGO observatory features L-shaped interferometer arms stretching around 4 km (2.5 miles), significantly longer than the Klystron Gallery, though some argue they should not count due to their design.
