Scott McCoy, age 10, of Enid, Oklahoma, for his question:

Which is the world's longest mountain range?

Mountains, you would think, belong on the continents where they can lift their lofty peaks to scrape the sky. For ages, they were the only mountains that people knew about. Even the geography experts tended to forget that we live on a watery world. No doubt they wondered about the worldwide ocean. But until modern times, nobody suspected that there are mountains, mighty mountains, under the sea.

The major mountains of North America range along the East and West sides of the continent. The ancient Appalachians slope down and dip their toes into the Atlantic. The Western mountians are the Rockies, the Sierra Nevada and the Coastal Range of California. Together, they form an enormous hump in the earth's crust, reaching all the way from Alaska to Panama.

There, in Central America, our mightiest mountain range links its fingers with the lofty Andes. Actually, all these Western mountains are part of the same geographical feature    an enormous range that stretches some 8,000 miles from the Arctic to the Antarctic regions. The enormous Himalayas hump their shoulders in Asia and lift up to the world's highest peaks. But the mountains along the Western side of the Americas form the longest range on any continent.

Now let's probe the oceans to measure the mountains that stand on the floor of the sea. We know now that the solid seabed has. ups and downs, like the dry land. Oceanographers have been making maps of it for several decades, but the big job of charting it all is not finished. One of the first features they noticed was the MidAtlantic Ridge, snaking down the very middle of the Atlantic Ocean. This underwater mountain range is at least 2,000 miles longer than the longest range on dry land.     Further probing revealed that the Paid Atlantic Ridge is merely one arm of a much longer system of mountain ranges in the sea. This big mid ocean ridge snakes through the major oceans for a distance of 40,000 miles. And so far as we know, it is the longest mountain range on this planet. The Atlantic Ridge begins near Iceland, runs down the very center of the North and South Atlantic and curves around South Africa. The East Pacific Rise ranges out to sea off our West Coast. It swoops around South America and Australia, and the two great ranges meet under water in the Indian Ocean.

For most of the way, the crest of the world's longest range is submerged under a mile or so of ocean water. From start to finish, the center of the ridge is split apart by a huge rift, or ditch, about 30 miles wide. Here and there the highest peaks poke up and form islands in the oceans. And also here and there, the great underwater range quakes and erupts hot lava onto the solid floor of the sea.