Hot fluids are nearly ubiquitous in volcanic environments in the Earth’s crust. Magma at depth heats groundwater which then ascends towards the Earth’s surface through faults, fractures, and otherwise permeable rocks. Fluids in geothermal systems offer direct insight into the many complex chemical and physical processes that occur in these extreme environments. They are also analogues of many ore-forming systems. Scientists have advanced our understanding of fluids in geothermal systems by studying wells sunk ~2–3 km deep into many geothermal fields. Today, we are targeting deeper and hotter reservoirs, at or near the contact of magmatic bodies, which provide unique opportunities to study, and potentially utilize, super-critical fluid resources in the near future.