Some immense hydrocarbon deposits arise when natural gas (methane) combines with water as a solid. Known as gas hydrate, this curious type of ice commonly forms under the seafloor, where in many parts of the world there is an abundance of methane and the appropriate pressure and temperature conditions prevail. Gas-hydrate deposits can also be found on land, under areas of permanently frozen ground. It remains unclear whether suppliers will ever try to tap these vast reservoirs of natural gas, because concentrations may be too low to merit extracting it. The authors explain how oil gas companies can take the first step toward probing the richness of hydrate deposits by deploying in boreholes a sensor that employs nuclear magnetic resonance.