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The main area of Sulyya Springs, the Subterranean Lake is filled with goodies to collect. Once you reach the plus-shaped platform, head to the water's edge and examine the highlighted area for a cutscene. After doing so, two small optional islands will appear, both containing a pair of Treasures and a number of guardians. These guardians are the most difficult encounters in the area, so if you can't handle them, come back later.