The past decade has seen a rise in the use of liquid cooling due to its energy efficiency. While many previous works have helped make progress toward improving data center cooling, a vast majority of them perform studies on a small system over a short span. The computer systems and HPC community lacks a long-term study highlighting the challenges and solutions in operating a liquid-cooled large-scale data center. We conduct the first detailed characterization of a petascale supercomputer, Mira, over a span of six years. The study is enabled by systematic monitoring of the environmental metrics, and discusses new research avenues, including coolant monitor failures.