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.
What are they? Ocean trenches are steep depressions in the deepest parts of the ocean [where old ocean crust from one tectonic plate is pushed beneath another plate, raising mountains, causing earthquakes, and forming volcanoes on the seafloor and on land.