The Underground Lake is a location on Excelcion and the setting of M07: Cauldron, in Metroid Prime: Federation Force. Prior to the mission, it was a Space Pirate testing area, where they were experimenting with size augmentation technology that would also allow them to control the subject they enhanced. A terminal found before the lake states that they abandoned it once their experiments were complete. True enough, no Pirates are encountered during the mission.
The experiments were conducted on a pair of Sawkens, massive, aquatic serpent-like creatures dwelling within the lake. A Federation survey team was sent to investigate, but the G.F.S. Aegis lost contact with them shortly after receiving an emergency distress beacon from the team. The Federation Force is sent to locate them, and finds them on the other side of the lake, alive. After battling the Sawkens, the team is saved and able to report their findings.
The lake has been frozen over, but when the Sawkens emerge, they break through the ice. A floating platform appears, possibly left over from the Space Pirates, and this is what the Federation Force stands on while battling the Sawkens.
"Pirate Log: Underground Lake
All experiments have been completed. This area is of no further use to us."