What scientific observation first suggested the presence of substantial deep water?
Answer
Anomalies in seismic wave behavior
Scientists monitored seismic waves generated by earthquakes; changes in their speed and behavior indicated the presence of dissolved materials, like water, at that depth.

Related Questions
Where is this vast, hidden reservoir of water located deep within the Earth?In what state is the water found in this deep reservoir?What specific mineral is identified as key to holding water in the transition zone?What scientific observation first suggested the presence of substantial deep water?What percentage of water by weight can ringwoodite in the transition zone hold?How is the deep water reservoir conceptually different from surface oceans?What geological process allows this deep water to potentially move over millions of years?What is a major implication of this deep water storage on planetary geology?Which laboratory or facility is mentioned in connection with confirming the theoretical capacity of ringwoodite to hold water?When visualizing the deep reservoir, what is the most significant difference from the common perception of an 'ocean'?