Extreme Mars 
The difference in height between the highest spot on Earth -- Mount Everest -- and the lowest spot -- the Challenger Deep -- is about 12 and a half miles. The planet Mars is only about half as big as Earth, but its extremes are more...well, extreme -- a difference of more than 20 miles.
At the top of the scale is Olympus Mons, a volcano that covers an area the size of Missouri. It's so big that if you stood at its base, you couldn't see the summit -- even though the summit is about 16 miles above the average elevation on Mars -- three times taller than Everest.
Olympus Mons is probably billions of years old. It's quiet now, but there's evidence that it's erupted within the last few million years. That means it might not be dead -- it could awaken at any time. There's also evidence that pockets of water could be trapped beneath the mountain. Warmed by the volcano's heat, these pockets could be inviting homes for microscopic life.
At the low end of the Martian scale is Valles Marineris -- a series of canyons that's long enough to stretch across the entire United States. In places, the canyons are more than a hundred miles wide, and more than six miles deep -- about six times deeper than the Grand Canyon here on Earth.
Valles Marineris probably formed as the Martian crust spread apart. Erosion by wind and water scoured it even deeper, creating the massive complex that's visible today -- the deepest point on an extreme planet.
Script by Damond Benningfield, Copyright 2009