Jeffrey Gerland, age 12, of Drexel Hill, Pennsylvania, for his question:

Where is the world’s greatest canyon?

Most people expect the world’s deepest canyon to be on one of the continents. In this case, it would be Grand Canyon in our western mountains. This one and several of its sister canyons are a mile deep and their rims are from eight to 18 miles wide. However, three quarters of our planet’s surface is under water. We tend to forget that the solid sea bed also has geographical features.

The shores of the continents slope gently downward under the water, then plunge over steep cliffs to the floors of the ocean basins. Many of these submerged cliffs are cut with steep canyons, deeper and even wider than Grand Canyon. In many places farther out to sea, the floor of the ocean is sliced with deep ditches. In the Pacific, one of these trenches dips almost three miles below the solid sea bed    and six and a half miles below the surface waves. It may be the world’s champion. But in any case, the world’s deepest canyon certainly is under the sea.