Although occupied submersibles have played an essential role in the discovery and study of hydrothermal ecosystems at depths ranging from 1500–3500 m, operational constraints at great depths have meant that the wide chemical and thermal diversity of the hydrothermal environments have long remained poorly defi ned. In the last decade, use of remotely operated vehicles (ROVs) to substantially extend dive time and development of a new set of dedicated instruments have greatly expanded our capacity to characterize seafl oor hydrothermal habitats at the interface between hydrothermal fl uid and seawater. In particular, major breakthroughs in the fi eld of in situ chemical sensing and highpressure experimentation have led to a much better understanding of the adaptation of invertebrate species to their extreme environment.