Poisonless replied. These are beings of Jambudvipa who did evil deeds. They have just died and passed through forty-nine days without any surviving relatives doing any meritorious deeds on their behalf to rescue them from their distress. Besides that, during their lives they themselves didn’t plant any good causes. Now their own karma calls forth these hells. Their first task is to cross this sea. Ten thousand yojanas east of this sea is another sea in which they will undergo twice as much suffering. East of that sea is yet another sea where the sufferings are doubled still again. What the combined evil causes of the three karmic vehicles evoke is called the sea of karma. This is that place.

(The Sutra of the Past Vows of Earth Store Bodhisattva)