Reach the bottom of the city was a mission that Jak and Daxter completed to obtain a power cell in The Precursor Legacy. After having explored much of the lost Precursor city they finally reached the very bottom, where Jak took a power cell and activated an old dark eco silo.
After the room with the power cell from Raise the chamber, head left into another tunnel with a smaller room. Here is the second sliding tube. Make your way through it and at the end you'll find the power cell situated on a strange platform in the middle of a Dark Eco silo. Take the Cell and activate the silo causing the Dark Eco to rise up quickly.
This mission directly leads into; Climb the slide tube.