Flexi Says: The Challenger expedition of the 1870s discovered a particularly deep part of the ocean, which they named Challenger Deep. That area is now known to be part of the Marianas Trench. Ocean trenches weren't well mapped or understood until much later. In the 1940s, ocean floor mapping advanced due to submarine activity in World War II. Studies of the ocean floor continued through the 1950s and 1960s. During this time, geologists began to understand the significance of deep ocean trenches as features related to subduction of tectonic plates at convergent boundaries.