Return to the dark eco pool was a mission that Jak and Daxter completed to obtain a power cell in The Precursor Legacy. When Jak and Daxter returned to the dark eco pool from the beginning of the game they found yet another power cell.
The PlayStation 3 and Vita versions of this mission are related to a trophy named "Tonight's Featured Event", completing this mission will reward you with a bronze trophy.
The way to the dark eco silo is as follows; enter the beginning area, proceed to the right right and proceed into the area with the many stone pillars. Beware of the bones that break away when you walk over them and the bone armor lurkers that patrol here. Cross the gaps moving towards the Precursor door at the end. Take the blue eco and enter the door.
You'll stumble right into an ambush, which consists of a lot of babaks and some bone armor lurkers. The Silo is very large and should provide plenty of space for Jak to run away, just take care not to enter the middle as a Cannon will be blasting that part continuously (unless you've already taken care to stop the cannon before entering the silo).
Although none of the lurkers are particularly difficult, try not to get swarmed, make sure to chain your attacks (punch, spin kick, uppercut, etc.). The Lurkers will drop red eco at times which makes it a lot easier, in this case your area of effect damage will be increased and you will kill any lurker in one hit. After defeating all the lurkers you can jump the steps and take the power cell atop the wooden walkway above the dark eco pool from the intro cutscene.