8,851.8 km

China, The great wall 

Three postcards of China's great wall, all new unwritten, it was package of 5 cards but i got this three from friend, who visited china few years ago,

紫禁城 - Forbidden City

Forbidden City, China 14-8-2010

Private swap with Xu, still didn't get info did he received mine card, hope he did.
As you can see on very bad picture it's a watch tower of Forbidden city, a center of ancient China. The Forbidden City was the Chinese imperial palace from the Ming Dynasty to the end of the Qing Dynasty. It is located in the middle of Beijing, China, and now houses the Palace Museum. For almost five hundred years, it served as the home of emperors and their households, as well as the ceremonial and political centre of Chinese government. Built in 1406 to 1420, the complex consists of 980 buildings with 8,707 bays of room. he Forbidden City was declared a World Heritage Site in 1987, and is listed by UNESCO as the largest collection of preserved ancient wooden structures in the world. FORBIDDEN CITY

长城 - Chángchéng

Great Wall of China 14-11-2009

I'm sure that everyone heard about this wall but here is few words about it. The Great wall of China  is a series of stone and earthen fortifications in northern China, build to protect north borders of the Chinese Empire against intrusions by various nomadic groups such as the Xiongnu from the north and rebuilt and maintained between the 5th century BC and the 16th century.ce the 5th century BC, several walls have been built that were referred to as the Great Wall. One of the most famous is the wall built between 220–206 BC by the first Emperor of China, Qin Shi Huang. Little of that wall remains; the majority of the existing wall was built during the Ming Dynasty. The Great Wall stretches from Shanhaiguan in the east to Lop Nur in the west, along an arc that roughly delineates the southern edge of Inner Mongolia. The most comprehensive archaeological survey, using advanced technologies, has recently concluded that the entire Great Wall, with all of its branches, stretches for 8,851.8 km (5,500.3 mi). This is made up of 6,259.6 km (3,889.5 mi) of sections of actual wall, 359.7 km (223.5 mi) of trenches and 2,232.5 km (1,387.2 mi) of natural defensive barriers such as hills and rivers.
And some scientist from china said that secret of wall's quality and long life is in stikcy rise thats is used to keep huge stones, with some other materials.  more about china wall  at wiki here.
This postcard is last one i recived on postcrossing.com  thats nomber 8. cause is toped with official sending and reciving cards for some time

China

Mr. Yin's Villa, Xinba Town, China, 8-11-2009

postcard recived on official postcrossing site, from user kellyim ,