The Swamp, also originally known as the Swamplands, is a secret location in Outworld and an arena shown in the Krypt and Extras kontent gallery special features of Mortal Kombat: Deadly Alliance, and appeared as a playable stage in the GameBoy Advance version of the game, Mortal Kombat: Tournament Edition.
This dank wilderness with mucky waters, strange crocodiles, and the bones of a long dead dinosaur, revealed to be a locale and part of the forgotten realm of Zaterra, Reptile's homeworld which explains the origins of both the crocodiles and the dinosaur skeleton. Since this area is part of Outworld, it is because of Shao Kahn conquered the Saurians' own home and merged it with Outworld long ago.
The Swamp was going to be an arena in Mortal Kombat: Deadly Alliance but it never made it into the game's production, thus it became a scrapped idea and picture of itself within the Krypt and Extras kontent gallery. Perhaps ironically, the arena was used in the portable versions.