Hydrocarbons play key roles in atmospheric and biogeochemistry, the energy economy, and climate change. Most hydrocarbons form in anaerobic environments through high temperature or microbial decomposition of organic matter. Subsurface microorganisms can also 'eat' hydrocarbons, preventing them from reaching the atmosphere. Using a new technique, scientists show that biological hydrocarbon degradation gives a unique biological signature. These findings could help detect subsurface biology and understand the carbon cycle and its impact on climate.