Tim Eylen, age 11, of Tacoma, Wash., for his question:

WHAT IS BENEATH THE OCEAN?

Far from land, on a ship out there in mid ocean, the shining waves reach out on all sides to where the dome of the sky dips down to the horizon. The water below the ship may be two miles deep or more. And it rests on some fabulous geography called the ocean floor.

The earth's crust is a rocky skin, ranging in thickness from five to 40 miles. This crusty layer is cracked into large and small plates that fit like a rather loose overcoat around the entire globe. Here and there, parts are uplifted to form massive continents and assorted islands. In other parts of the crust are enormous hollows.

These hollows are the basins that hold the waters of the oceans. One might expect the stupendous weight of water to crush and level the floors of the seas. But the bed beneath the ocean is as bumpy as the land. The main reason for this is that these crustal basins are as restlessly active as the moutainous land areas, if not more so.

The average depth of the seas is about two miles. It ranges from a few inches along the tidal shores and dips gently down the continental slopes. These end where the continental shelves plummet down the steep walls of the deep ocean basins.

Here and there the floor of the abyss stretches in flat plains, carpeted with silty clays and layers of ooze formed by age old accumulations of diatoms and other sea dwellers. On these plains there are lonely submarine mounts, like old volcanic cones. There also are strange flat topped underwater mounts called guyots. These too may be old volcanos that surfaced when the sea level was lower  and pounding waves bashed off their pointed heads.

In addition to the plains and lone mountains, there are enormous cracks or trenches, plunging several miles below the surrounding sea bed. However, the most spectacular undersea features are mountain ranges, stretching down the major oceans.

They are longer and more massive than the largest land ranges. These ranges are called mid ocean ridges. Most of them straddle cracks in the crust, where lava material oozes up from the mantle below. This new material constantly spreads the crustal plates apart. We now know that this sea floor spreading explains how the continents drift apart.