The Zangar Sea (also known as the Sea of Zangar) is the name of the sea that lies between Frostfire Ridge and Nagrand on Draenor. Seemingly having a mind of its own, it seeks to overtake the land by releasing thousands of spores and slowly growing inland over time. The sea, and thus the Zangar encroachment, was created from the rotting body of the Sporemound Zang.
Marshes covered in fungi existed along the coast. At some point, either because of fel corruption, because of Draenor shattering, or because of the aforementioned intention to overtake the land, the rest of the sea was transformed into a swamp that is known now as the Zangarmarsh.
Concept art of a fungal whale.