http://www.newsweek.com/reason-mysterious-gaps-great-wall-china-revealed-archaeology-history-1857564

Archaeologists studying a little-known stretch of the "Great Wall of China" have revealed new insights into the history of what is considered one of the largest engineering projects ever undertaken.

A team of researchers investigating one section of the defensive structure identified several large gaps. They argue these gaps could indicate that parts of this section were never finished—a sign that the structure was hastily built under the threat of invasion from Mongol armies in the medieval period, according to a study published in the Journal of Field Archaeology.