© Agence France-PresseIn this lithograph, Roanoke Colony settlers are shown baptizing Virginia Dare, the first child born in the New World to English parents.
Twice in American history, entire groups of people have suddenly disappeared - to the point that archaeologists and anthropologists today cannot say where they went or what happened to them.

One was the first boatload of English settlers in the New World. In 1587, they established a colony on Roanoke Island, off the Atlantic coast of what is now the state of North Carolina. The settlement had great difficulties with food and hostile Indians, so its governor went back to England to get supplies and arms.

When he returned, the whole colony of about 100 people had vanished without a trace. All that was left in what became known as the Lost Colony was the word "ROATAN," carved into a tree. The Roatans were Indians in the area.

The second mystery evolved several hundred years earlier, and several thousand kilometers to the west, in what is now the state of Colorado, past the Rocky Mountains in a place called "Mesa Verde" - "green table" or tabletop in Spanish.

A mesa is a long, flat-topped mountain, rising above a valley. About 1,900 years ago, this one was the home of a native people that today's Navajo Indians call the Anasazi - the ancient ones.

They were peaceful Stone Age bean farmers, hunters, and basketmakers. At first they lived in pit houses sunk into the top of the mesa. Then, about the year 1250, they climbed down the sheer cliffside using perilous hand- and toeholds, and began living among the rocks. To ward off the boiling sun and howling Colorado winds, they carved chambers for shelter and ceremonies.

One can see the largest cluster of chambers, now called "Cliff Palace," which housed about 200 people. There was water there, seeping through rocks into pools in the chambers.

As you can imagine, enemies had a hard time pursuing them there, so the Anasazi would seem to have been safe from attack.

But within only 50 years or so of moving in, they disappeared from Mesa Verde, which is now a U.S. national park.

Why did they leave, and where did they go? No one knows for sure, but unlike the case at Roanoke Island, there's a clue: There is evidence that a terrible drought set in. Archeologists believe they simply left for greener mesas and blended into neighboring tribes.