Jean Leffler, age 12, of Wauwatosa, Wisconsin, for her question:

What exactly is the Mariana Trench?

Gardeners dig ordinary trenches to set out the spring vegetables. The earth, however, has taken her time to create some extraordinary trenches    in the most unlikely places. So far as we know, the grand daddy of them all is the Mariana Trench, an immense ditch in the deep floor below the Pacific Ocean. If the tallest mountain in the world decided to walk over and stand at the bottom of this stupendous ditch, its snow capped summit would be more than a mile below the waves.

For centuries, sailors sailed the seas with little idea of what was down there below. the waves. Some never dreamed that the ocean waters rest on a stolid floor, two to three miles below the stormy surface. Ignorance prevailed because unaided divers could descend only about 100 feet. Nowadays, helmeted divers with special breathing apparatus and the best of modern equipment can probe to 600 feet. This is the average depth of most submarines. But in January 1960, The Trieste, an ocean probing capsule, descended almost seven miles and stayed for 20 minutes down in the Mariana Trench.

Probing the dark mysteries of the deep ocean had to wait for the invention of echo sounders, specially designed ships, submarine capsules and other modern devices. In the past few years, much of the ocean floor has been charted, but the patient work is far from completed. The first probes were full of surprises, for the ocean bed has more noteworthy geographical features than the dry land. Among these submarine ups and downs are hills and lonely, flat topped mounts called guyots, vast plains carpeted with thick, oozy layers of silts and clays. There,,,also are long fractures and several stupendous ditches called trenches.

In 1951, HMS Challenger echo sounded to a record depth and located the abyssal Mariana Trench, about 250 miles southwest of Guam Island in the Pacific. Oceanographers of other nations returned to investigate in more detail. Their measurements gave depths of 35,460, 35,631, 35,800 and 36,198 feet. These variations suggest that the floor of the ditch is uneven and dented with pits.

If Tim's Hill, the highest point in Wisconsin, stood in this watery depth, the surface waves would toss almost seven miles above its handsome head. Even if the earth's loftiests peak, Mount Everest, stood in the trench, the ocean surface would be more than a mile above its crown.

The awesome Mariana Trench and three similar trenches lie along the western side of an immense submarine mountain range. Its crests break the surface and form the Aleutians, the Philippines, New Zealand and the islands of Japan. In this restless zone, the earth's crust has created steep ridges alongside its deepest plunges.

Down in the midnight depth of the trench, the water exerts a pressure of 7 1/2 tons on every square inch. Surely no living things can exist in such a realm! But they do. In 1960, The Trieste revealed proof of a variety of living things at a depth of 35,800 feet. This plunging ditch below almost seven miles of water is home to certain mollusks and crustaceans, assorted polyps and echinoderms.