Dirty work

How long would it take to fill the Grand Canyon with dirt? About 400 years if you used all the soil shuffled around by humans in the U.S. every year. Compare that with the 5 million years it took the Colorado River to carve out the canyon.

Humans shovel earth to build cities and retrieve valuable minerals from underground. Roger LeBaron Hooke, a geologist at the University of Maine, calculated that people probably move more dirt than all the natural earth-movers--like rivers and wind--combined.

But altering landscapes can have unintended consequences. For instance, it can increase the washing away of rich soils needed to grow food, or even cause rivers to overflow their banks. "As our earth-moving ability increases," says Hooke, "so does the potential for catastrophes."

FILL IT UP: The Grand Canyon snakes across 446 kilometers (277 miles) with an average depth of 16 km (1 mi).

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