Alexandra Votten, age 11, of Costa Mesa, Calif., for her question:

WHAT ARE OCEAN TRENCHES?

Very long and deep valleys on the floor of oceans are called trenches. Challenger Deep, located in the Mariana Trench of the Pacific, if the deepest known spot in all the oceans. It is 36,198 feet below the surface of the sea.

Many of the ocean trenches are covered with layers of mud, leveling parts of the ocean bottom. Some of the mud covered trenches are full of shells and skeletons of dead animals and decomposed plants.

Scientists call the mud found in the ocean's trenches ooze. They believe that most of the oozes have been formed over millions of years. They may actually be hundreds of feet deep in places. Beneath them lies a hard rock base called basalt.