The standard cosmological model, Lambda-CDM, provides a remarkably successful description of the universe, yet the physical nature of dark energy remains unknown. Is it a cosmological constant, or does it evolve with cosmic time? The Dark Energy Spectroscopic Instrument (DESI) was built to address this question by obtaining spectra of millions of galaxies and quasars and constructing an unprecedented three-dimensional map of the universe. In this colloquium, I will tell the story of how DESI became one of the world’s most productive instruments for cosmology. At its heart are 5,000 robotic fiber positioners, built at the University of Michigan, that allow DESI to observe thousands of objects simultaneously. I will tell the story of how an ambitious idea became a working instrument - from construction at Michigan through integration and installation to survey operations at Kitt Peak. Along the way, DESI faced a global pandemic, a wildfire that threatened the observatory, and other unexpected challenges of operating a complex instrument on a mountaintop. Despite these obstacles, it reached its survey goals ahead of schedule and within budget. I will discuss what made that success possible, place DESI’s emerging cosmological results in context, and conclude with the scientific opportunities ahead.