The ocean trenches are narrow topographic depressions in the sea. These trenches form the deepest points in the ocean and are the lowest points on earth.
These trenches are formed at a particular place of plate boundary which is known as subduction zone. It happens to form when a plate in ocean hits another ocean plate or it strikes with continental plate. Oceanic plates are made of basalt which is a denser rock while continental plates are formed up of granite which is not as dense as the basalt. Now when one plate hits another plate or continental plate in the ocean then the denser plate between the two of them sinks in the mantle. The place where this plate sinks will create a deep trench on that spot of ocean. They are associated with a chain which is known as island arc or the zones that experience frequent earthquakes. The trenches can be very long up to thousand of kilometers and they may be parallel to island arc volcanoes.
Some geologists say that the size of earth is same from many hundred million years but tectonic theory says that new crust is generated along the divergent plate to few centimeters per year. We know the earth size has not changed which implies that the roughly same amount of crust is destroyed as it is made. So this subduction along ocean boundaries can compensate with the new crust.
Trenches are narrow steep sided valleys. Trench water is cold because of temperature from zero degree C to two degree C. some factors that affect the depth of trench is supply of sediment which fills the trench, age of lithosphere at time when its subducted etc. The deepest point in the oceans of planet is The Challenger Deep in Mariana trench of Pacific Ocean. The point is 10924 miles deep and is named after British ship Challenger 2 that discovered this spot in year 1951. If the highest mountain of our planet, Mount Everest is placed on it then a mile of water will remain over the topmost point of the mountain.