The oceanic trenches are hemispheric-scale long but narrow topographic depressions of the sea floor. They are also the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few mm to over ten cm per year. A trench marks the position at which the flexed, subducting  slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about from a volcanic arc. Oceanic trenches typically extend below the level of the surrounding oceanic floor. The greatest ocean depth to be sounded is in the Challenger Deep of the Mariana Trench, at a depth of below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3 km²/yr.