英语听力：自然百科 探索世界奥秘之万里长城 Unit6（在线收听）

探索世界奥秘之万里长城 Unit6

It is a staggering 2,700 miles longer than the first Great Wall of the Emperor Qin. There is no wood, water or stone in the desert. But the Han managed to build a wall that has resisted 2,000 years of erosion. You can still see the layers constructed with only local materials, layers of compressed sand, twigs, bark, tamarisk and horse manure. The wall has withstood conditions that alternated between torrid heat for half the year and freezing winds for the rest. Building this wall in the middle of the barren desert was only half the task. It had to be manned. Soldiers were constantly told to be vigilant and not to move an inch from their posts. This was particularly true of those manning these towers, built miles in front of the actual wall. Despite millions of soldiers in the army, the wall was so long they didn't have enough men to post a sentry every few yards, so plenty of advance warning from these towers was crucial.

Using smoke signals, the Chinese evolved a highly sophisticated and successful signaling system. A beacon lit here in the Gobi can pass its message some 700 miles in only 24 hours, alerting the entire frontier to the danger.

"Chariots and horsemen are directed to keep on guard and the men at the watchtowers to keep a sharp lookout for fire signals and keep the border clear." says a Han military document. Each beacon was constructed within clear view of its nearest neighbor so that fire signals at night or smoke in the day could be seen and the message passed on. And this was their most precious commodity in the barren desert. It is firewood, used to light the ancient signal fires. It's still here where it was abandoned some 2,000 years ago. We know so much about these ancient lives because the dry Gobi Desert has preserved their letters, buried next to the watchtowers where they lived and often died.

Dear all, in this far-off labor camp, I miss clear grounds in the middle of the sands.

This is no camp. It's like a tomb. And I miss you so much.

Last week, I received a punishment.

I can't express properly (coldest . . .) my gratitude toward you, dear elder brother.

Do you remember that night? (Dear old parents) We drank so much.

Thanks for the food and medicine you sent to me. Words can't translate what my heart feels for you. There is only something for you…

Today, we made 150 bricks . . . 

We all look like slaves.

The moon is shining through a sea of clouds. When we are on guard, all we can see is this desolate landscape, devoid of any sign of life. The memory of our home become so painful. Our exile seems as though it will never end.