The Lost Sea is a body of water surrounding Shang Tsung's Island. A ship transports contestants from other locations to the island. The Lost Sea's exact location is unknown. However, it is situated somewhere between Earthrealm and Outworld.
The Lost Sea is mentioned in the first Mortal Kombat. In Mortal Kombat 11, Jacqui Briggs and the past version of Jax Briggs traverse the Lost Sea to reach Shang Tsung's Island and retrieve Kronika's Crown. The waters around the island were shrouded in green mist and the wreckage of numerous ancient vessels could be seen.