In the ponderosa pine forests and deserts of northern Arizona, the Colorado River undulates for 277 miles through the red-banded rocks of the Grand Canyon. Millions of people visit the canyon each year, but few know about the region’s other geologic rarity: uranium-rich columns of rubble. Some 200 million years before the Grand Canyon existed, subterranean water hollowed out pockets in the Colorado Plateau. Over time, the rock layers above collapsed into these empty spaces, creating underground shafts, called breccia, or “broken,” pipes, approximately 300 feet in diameter and up to 1,000 feet deep. Their rubble pulled uranium from groundwater — meaning that the pipes hold some of the nation’s richest and most accessible uranium deposits...