During shows followers will flail their cellphones and light sticks since the artist(s) performs, generating an "ocean" of light from the viewpoint of performer. A "black ocean" is a trend in K-pop (Korean Pop or music) where in fact the audience turns off their particular light sticks and cellphones as a show of lack of help to the present group onstage. When done collectively, it seems towards the performers as a "black ocean" as a result of the decreased lights from the audience. Generally just takes place in acute cases. However, "extreme" is a subjective word when it's put together using craziness and stupidity that's the hardcore fans of K-pop.