陥没 ("subsidence", more generally a depression or recess) is probably a euphemism for inverted nipples. 沈頭鋲 is a little strange, though. It appears to be a construction/manufacturing term, meaning "flush rivets", applying it to nipples seems novel or at least unusual.
Di Molto! Wonderful! It's the most dense forest!Aaah! It's subsidence... No, embedded tack!Ah, No..!Let it grow like this.
Your big sister is sad and super happy at the same time!!Yeah Yeah~ ♡ Come out~♡ STOP IT GUYS! THE ADMIRAL IS WATCHING US!