China's Great Wall filmed by drone
5 January 2017 Last updated at 23:59 GMT
British geographer, conservationist and author William Lindesay has had a lifelong obsession with the Great Wall of China.
Three decades ago, he left his home on Merseyside to live near the wall so he might better be able to study it.
In 2016 he and his family travelled 15,000km (9,320 miles) around the wall network, filming it from the air with a drone.
Mr Lindesay and his sons, Jim and Thomas, spoke to the BBC about their epic journey and how they shot it.
Read their full story here: One man's mission to walk Great Wall
Footage by James and Thomas Lindesay at Depictograph.
Produced by Anna Jones and Simeon Paterson