Carolyn Billington, age 11, of Huntington Beach, California, for her question: 

Which is the world's biggest volcano?

The roots of a volcano reach miles, often many miles below the ground. We cannot measure how deep they go or how wide they spread. But we can measure the height of a volcano as we measure the height of a mountain. In fact, some of our tallest mountains are volcanoes with a fiery past. But their violent history is over and they will never again erupt with a flaming fire. We want an active volcano and the world's tallest active volcano is found in Ecuador. It is steamy Cotopaxi, 19,344 feet high, and it erupted as recently as 1942.

Many volcanoes stand on the land and we can measure their height above the level of the sea. But the islands of Hawaii are really volcanoes that stand on the floor of the sea. The ocean floor is almost four miles below the dry land. The smouldering crest of Hawaii's Mauna Loa reaches another 13,680 feet above the water. Measured from its base on the ocean bed, it is the biggest volcano in the world.