Unblock the eco harvesters was a mission that Jak and Daxter completed to obtain a power cell in The Precursor Legacy. Samos Hagai's green eco harvesters at Sentinel Beach had been blocked with large boulders, he asked Jak to unblock them.
Upon entering Sentinel Beach there are a multitude of ways to reach the blocked eco vents at the far end of the beach, on top of the elevated area on a series of platforms nestled in a corner nestled among the cliff wall. Use either the platforms attached to rotating wheels, a high jump or any other natural steps to reach the higher grassy plateau area and escape the cannon on the beach. Once on top, keep walking while taking care of any lurker puppys that will attack you.
Eventually you will run into a set of five large balloons, which are the aforementioned eco harvesters. Each balloon is placed above a green eco vent, each of which are in turn blocked by a large rock. Hit each to release the eco inside the vent, ensuring a constant source of green eco for this region in case of an emergency, the last rock will release a power cell upon being broken.