The Sea of Blood

The Sea of Blood is the area full of "high concentration" red Shinsu (hence why it's become liquid like water), that covered the real view of Floor of Death from sight.

About
This "red Shinsu" is apparently the blood of the deceased 43rd Floor Guardian. You can't use shinsu when you're diving-in or swimming inside this blood, not even High Ranker like Princess Ha Yuri Zahard.

Notes and Trivia

 * You can't use shinsu inside the sea of blood.