Direct-liquid (i.e., water) cooling is the de-facto standard for cooling large-scale High Performance Computing (HPC) systems. While the idea of mixing electronics with water does not appeal to everyone, the superior cooling properties of water compared with air do: water cooling facilitates lower CPU temperatures, more compute performance, lower hardware failure rates, and higher power densities. Additionally, it can increase energy efficiency, as it can sustain safe operational conditions at significantly higher temperatures than air could. That in turn allows for chiller-less cooling year-round in most climate zones and potential re-use of the waste heat. This talk will introduce and explain the benefits of direct-liquid cooling, its application in HPC data centres and its impact on green IT operations.