As the scalding hot fluid comes out of a vent, it mixes with the cold seawater, cools significantly, and is moved upward and spread out by ocean currents. This mixed water ends up only a fraction of a degree warmer than the surrounding seawater. The key to locating this plume is to measure the ocean temperature continuously, in the hopes of finding some temperatures (called anomalies) that are slightly higher than normal. Imagine pouring a cup of boiling water into a swimming pool. The pool water will become a tiny bit warmer in that spot, and this can only be detected by using a really good thermometer in just the right place.

On board, computers display continuous temperature readings in the form of graphs, which scientists study for temperature anomalies. Even very small differences are significant. There might be only one part of hot hydrothermal fluid (at 350°C) diluted in 300,000 parts of seawater (2°C), so a difference as small as .001°C is enough to warrant a closer look.