根据以下材料，回答题
Deep inside a mountain near Sweetwater in East Tennessee is a body of water known as the LostSea.It is listed by the Guinness Book of World Records as the world' s largest underground lake.TheLost Sea is part of an extensive and historic cave system called Craighead Caverns.
The caverns have been known and used since the days of the Cherokee Indian nation.The caveexpands into a series of huge rooms from a small opening on the side of the mountain.Approximately onemile from the entrance, in a room called "The Council Room", many Indian artifacts have been found.Some of the items discovered include pottery, arrowheads, weapons, and jewelry.
For many years there were persistent rumors of a large underground lake somewhere in a cave, butit was not discovered until 1905.In that year, a thirteen-year-old boy named Ben Sands crawled througha small opening three hundred feet underground.He found himself in a large cave half filled with water.
Today tourists visit the Lost Sea and ride far out onto it in glass-bottomed boats powered by electricmotors.More than thirteen acres of water have been mapped out so far and still no end to the lake hasbeen found.Even though teams of divers have tried to explore the Lost Sea, the full extent of it is stillunknown.
The Lost Sea is unique because it is________.