"Er" means "two" in Chinese, so perhaps it refers to four of whatever you were referring to
BTW the photography is just magnificent. It would be complete pitch darkness in the cave, so it looks as if the photographer brought down some battery powered studio flash with him. Nothing else would be powerful enough to illuminate the giant cave like that. He placed the flash very well too. If you study the shadows you can see where the light came from but it's not intrusive.