I was 13 when the incident happened. My mother had taken me for a holiday in Beijing. I think it was in the month of March.

That day was such a foggy day. The clouds were overcast and it was very cold about below zero degrees celsius. At that time, I was at the great wall of China. All of a sudden, I heard someone screaming. When I went near the wall, I heard it loud and clear. I heard knockings too. What's inside the wall?