Apparently the Precursor structure just off the coast leads to an underwater lost city!Keira, The Precursor Legacy
The lost Precursor city is a location in The Precursor Legacy. It is implied to be a long since-abandoned Precursor civilization: a massive underwater networking chamber, located via an elevator, and home to a large dark eco silo. Despite being deserted and infiltrated by lurkers, most of the machinery seemingly still functioned.
Before his disappearance, the Blue Sage had discovered the lost Precursor city off the coast of Rock Village and made unsuccessful attempts at bringing one of the underwater chambers to the surface. After Jak, Daxter, Samos, and Keira's arrival to Rock Village, Keira discovered the Blue Sage's endeavors, presumably through the journal which Samos found. Keira then tasked Jak and Daxter with the mission "Raise the chamber", in which Jak successfully raised an underwater chamber to the surface and gained a power cell. While inside the lost Precursor city, Jak completed various other missions and puzzles to gain more power cells to further his journey north.
The entrance to lost Precursor city takes on the form of a dome located off the coast of Rock Village. The underwater city itself is a complicated network with various passageways, platforms, rooms, and chambers. It was mainly constructed of Precursor metal and glass windows, allowing for the observance of marine wildlife, and is home to various ancient Precursor artifacts, complex puzzles, platforming challenges, as well as various types of lurkers. It is also littered with defense mechanisms such as energized bodies of water, pools of dark eco, scorching metallic pipes, and timed traps. It is guarded by lurkers who seemingly specialize in guarding designated areas such as the double lurker, puffers, and bullys.
Located via a large tube-like slide connecting a small room to the sea floor is a small complex which contains the chamber that Keira wanted raised to the surface. After channeling blue eco from the multitude of eco vents within the complex, Jak successfully activated the formerly dormant chamber and raised it to the surface to gain the power cell located on the chamber's roof.
As previously mentioned, the city is home to a massive dark eco silo. In the mission "Reach the bottom of the city", Jak accidentally activated a silo from stepping on a platform switch which contained a power cell. The dark eco levels in the silo quickly began to rise as Jak had to climb the silo using the circumventing platforms and blue eco launchers in order to escape.