Below the snow-covered peaks of the Andes Mountains, among scattered rocks and the scrub of prairie bushes, there sits at this very moment a 12-ton polyethylene tank holding 3,000 gallons of pure water.

All around it, spread out in every direction over an area nearly the size of Rhode Island, are 1,599 more such tanks, each identical to the first. These lonely sentinels have their eyes on the sky, patiently observing what human eyes cannot in the hopes of solving a mystery that began on another continent and more than three decades prior — a mystery that started with the Oh-My-God event.

It was the night of October 15, 1991. The University of Utah had set up an experimental observatory called the Fly's Eye in the isolation of Dugway Proving Ground, a sprawling 800,000-acre tract of land used by the U.S. Army to test biological and chemical weapons since the 1940s. On that night the Fly's Eye detected something called an air shower, a miles-long explosion of streaming particles invisible to the human eye and caused by high-energy interactions in the upper atmosphere. Each of the telescope’s detectors were designed to point at a different part of the field of view, in a similar way to insects’ compound eyes. It was this that earned the telescope its name. “We were hoping we might pick up something really unusual,” says David Kieda at the University of Utah, who worked on the telescope at the time. (Read more about the Fly's Eye Array here.)

Scientists looked at the data they'd collected and worked backward to deduce the properties of the space-borne particle that led to the air shower. The results weren't just shocking — they were thought to be impossible. They called their discovery the Oh-My-God particle.

While the Oh-My-God particle still remains the most energetic cosmic ray ever detected, a handful of others in the off-the-scales range have been observed in the years since, confirming that it wasn't a miscalculation or instrumentation failure, but in fact a real event. This is why 1,600 giant water-filled tanks have been installed in a grid formation across 3,000 square kilometers of the arid Mendoza region of Argentina. These are the specialized detectors of the Pierre Auger Observatory, forming an array designed to capture evidence of other extremely high-energy cosmic rays. "The quest for identifying the sources of the most energetic particles in the Universe continues," says Carsten Rott, chair of the Department of Physics & Astronomy at the U. "[But] not only at the Auger detector in Argentina, but also right here in Utah with the Telescope Array experiment."