[vanilla] Parasites near Great Wall

I don't know if this has already been talked about blah blah blah...

Sometimes in my games there were lines like parasites across the whole screen, but only at certain places of the map. I couldn't take a screenshot as it doesn't appear on them. I wondered where it came from, but lately it started to appear on my lands right after i built the Grat Wall. It can do it also at certain location, foreign civs that maybe built the great wall.

This may disapear with a reload though.