According to Science Daily, the most significant geological event that happens in ocean trenches is the gradual subduction of the Earth's crust down into its mantle. This process takes tens of millions of years.
Ocean trenches form when oceanic tectonic plates meet continental crust. The two plates exert tremendous pressure against each other, and eventually the lip of the ocean plate crumbles beneath the continental plate. As the plates continue to press against one another, the ocean plate gradually sinks beneath the continental crust and re-enters the mantle.
According to Live Science, the deepest known ocean trench is the Mariana Trench, located in the Pacific Ocean. It is approximately 1,500 miles long. The deepest portion of the Mariana Trench is the Challenger Deep, a narrow portion at the southern end, which is roughly 36,000 feet deep. The water temperature in this area is slightly above freezing.
The world's ocean trenches remain largely unexplored due to the immense pressures at such depths. For example, Live Science explains that the pressure along the bottom of the Mariana Trench is 8 tons per square inch. To survive in such conditions and avoid implosion, trench exploration vehicles need to withstand pressures equivalent to the weight of 50 Boeing 747 jets stacked on top of them.