The Great Wall of China is the longest man-made structure in the world but that doesn't mean it's known exactly how big it is.

A five-year archaeological survey done by the State Administration of Cultural Heritage (SACH) found that the total length of the Great Wall was 13,170 miles long and reached across 15 provinces, more than twice the length previously stated just two years ago, when the same organization reported that the wall was 5,500 miles and stretched across 10 provinces.

How could they be so far off?  It seems no one agrees on what the Great wall even is.  “The previous estimation particularly refers to Great Walls built in the Ming Dynasty (1368-1644), but this new measure includes Great Walls built in all dynasties,” Yan Jianmin, the office director of the China Great Wall Society, told the China Daily.

Now they think they have it locked down. The Great Wall counts everything built starting more than 2,000 years ago to ward off invasions and is one of the Seven Wonders of the World. Eight percent of the wall is still standing today.