71. How many lightning strikes occur worldwide every second?

On average, about 100. Those are just the ones that hit the ground, though. During any given minute, there are more than a thousand thunderstorms around the Earth causing some 6,000 flashes of lightning. A lot of it goes from cloud-to-cloud.

72. Are rivers alive?

Not in the traditional sense, of course. But like all living creatures, rivers have a life span. They are born, grow in size, and they age. They can even die during the span of geological time.

73. Can asteroids create islands?

Speculation has existed for decades that ancient asteroid impacts might create hot spots of volcanic activity, which could give rise to mountains that poke up through seas that didn't used to be there. There's no firm answer to this question, but a recent computer model suggested Hawaii might have been formed in this manner.

74. Is the state of Louisiana growing or shrinking?

Louisiana loses about 30 square miles (78 square kilometers) of land each year to coastal erosion, hurricanes, other natural and human causes and a thing called subsidence, which means sinking. Much of New Orleans actually sits 11 feet (3.4 meters) below sea level. Parts of the French quarter have sunk 2 feet in the past six decades. The city is protected by dikes, but all experts agree that storm tides from a direct hit by a major hurricane would breach the system and swamp much of the city. In 2000, the director of the U.S. Geological Survey, Chip Groat, said: "With the projected rate of subsidence, wetland loss and sea-level rise, New Orleans will likely be on the verge of extinction by this time next century."



75. How much would seas rise if the Antarctic Ice Sheet melted?

The Antarctic Ice Sheet holds nearly 90 percent of the world's ice and 70 percent of its fresh water. If the entire ice sheet were to melt, sea level would rise by nearly 220 feet, or the height of a 20-story building. Scientists know there's a melting trend underway. The United Nations has said that in a worst-case scenario -- depending on how much global air temperatures increase -- seas could jump 3 feet (1 meter) by 2100

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