Beyond these three uses, there is much disagreement over when to capitalize. Every major publication and publisher has its own standards, and many writers have their own preferences (which are generally overridden by in-house rules during publication).
Deep-sea trench. Deep-sea trench, also called oceanic trench , any long, narrow, steep-sided depression in the ocean bottom in which occur the maximum oceanic depths, approximately 7,300 to more than 11,000 metres (24,000 to 36,000 feet). They typically form in locations where one tectonic plate subducts under another.
Plate Boundaries. At the same time, some of the oldest ocean crust occurs in deep sea trenches, which run parallel to continental mountain ranges. A lot of very large earthquakes have been plotted along deep ocean trenches, suggesting that these are seismically active areas (meaning the crust is moving).