You're hiking through the Rockies one day when you hear a low, ominous rumble.   Landslide! Hundreds of tons of rock and soil are careening toward you at enormous speed.

Fortunately, there's a helicopter nearby and you lift off just in time to escape unharmed. But now you notice something odd. After the landslide has gone by, in its tracks little dainty flowers are still standing. Blades of grass are unbent. How is this possible?

Before we tell you the answer, here's another question. What happens when you drop a book flat onto a table?

If you try it, you'll see that the book doesn't hit the table unimpeded, but lands on a little cushion of compressed air. If you don't believe us, try putting some little pieces of paper on the table before dropping the book. The whoosh of compressed air escaping will blow them off.

That's the same phenomenon that happens with landslides. When an enormous sheet of rocks and dirt gets moving, it acts like a solid object. It traps the air underneath it and actually rides downhill on a cushion of air. Strange as it may seem, much of the landslide never touches the ground at all.

Because the cushion of air stays underneath it, the rocks and dirt aren't slowed down by friction much and continue picking up speed as they fall. Large landslides have been clocked in excess of 100 miles an hour.

So -- could you just lie down and let the landslide whoosh over you? No, unless you are about the size of a blade of grass. People who are bigger than that should keep a helicopter handy.