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.
Ocean trenches form some of the deepest locations in Earth's oceans. Mariana Trench earthquake: Map showing the location of the Challenger Deep, the epicenter of an April, 2016 earthquake, and the relative movement directions of the Pacific and Philippine Plates. USGS map with annotations by Geology.com.