The sole remaining river upon Mars, for ages green and red Martians upon their thousandth year of life would take a pilgrimage to the River Iss, they would travel down to the river to what they believed was the afterlife.
Those who did not have the courage to complete this journey would abandon their attempt and end up in the valley of lost souls, or upon the low islands that dotted the river. They would usually die, causing these small islands to be covered in their rotting corpses.
Those who complete their journey down the river find no paradise. At the end lay the lost Sea of Korus and there the plant men and white apes await them. If they do not meet their ends at the hands of the apes and plant men, they are likely taken by the Therns where they would be enslaved, or used as a food source.