根据以下内容，回答题。
Passage Four
Deep inside a mountain near Sweetwater in East. Tennessee is a body of water known as the Lost Sea. It is listed by the Guinness Bool of World Records as the world"s largest underground lake. The Lost 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 cave expands into a series of huge rooms from a small opening on the side of the mountain. Approximately one mile from the entrance， in 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， but it was not discovered until 1905. In that year， a thirteen-year-old boy named Ben Sands crawled through a 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 electric motors. More than thirteen acres of water have been mapped out so far and still no end to the lake has been found. Even though teams of divers have tried to explore the Lost Sea， .the full extent of it is still unknown.
The Lost Sea is unique because it is__________. 查看材料
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