The Swamp of Sorrows is a marshland in southern Eastern Kingdoms previously filled with green dragons serving Ysera (which could be farmed for the Tiny Emerald Whelpling). These days you can farm the whelps in Northern Feralas for the pet.
When the Dark Portal imploded during the Second War, The Black Morass was split in half – the northern part became the Swamp, while the southern half became the barren Blasted Lands. The Horde base of Stonard remains while the Alliance now have a town located on the lakeside near the Sunken Temple.
In addition, the coastline that once enclosed the Temple’s Pool of Tears is now open to the sea.