What do we do with particularly nasty wastewater? More often than most people realize, the answer lies beneath our feet. Injection wells (also called “disposal wells”) send waste from industrial processes and energy production into “thirsty” rock formations deep underground. This technology, mainly developed within the oil and gas industry, increasingly holds a key role in a range of clean energy and environmental engineering applications, from replenishing aquifers to generating geothermal energy to sequestering carbon. Recently, controversy has surrounded this little-noticed technology. In some areas, hazardous wastewater pumped into injection wells has contaminated aquifers, leaving local residents concerned about the safety of their water supply. And, as people in several states including Texas, Oklahoma and Arkansas discovered during the shale boom, dump too much heavy liquid underground and deep fractures can awaken. The largest earthquake in Oklahoma’s history—a magnitude 5.8 in the fall of 2016—has been traced to the disposal of wastewater from oil and gas production. Geothermal operations that rely on injection of water have been halted in Europe because of quakes they caused. Although disposal wells are widely deployed, very few people understand all the complexities of wastewater injection. Those who do rarely agree on the risks and benefits of injection wells, or even agree on the language to use in discussing them. Uncertainty over the risks of disposal wells has pushed projects into limbo—including geoengineering schemes that some deem necessary to avert climate disaster. In this two-day meeting, a range of experts and stakeholders will discuss insights from their experience with injection wells; the history of the practice; the economic, social, and environmental significance of injection wells; parallels and differences with other waste disposal and environmental engineering issues; and new injection well methods and applications. Our goal: develop a shared vocabulary and expertise around the questions of what, how, where, and whether to pump things underground.