The dark cave is a relatively well-hidden location in Spider Cave. Its entrance is in between the third and fourth dark eco crystals from the "Destroy the dark eco crystals" mission. Similarly, one can start at the small tunnel leading to the cavern with the Precursor robot, go right up to the scaffolding and while keeping the wall on your left you will eventually find the specific small cave to your left past a yellow eco vent.
Cross the lake and head into a unique section of Spider Cave: a large and mostly bottomless cavern. The only actual light source are large crystals which are hardly illuminated. However, hitting these with a melee attack will cause them to discharge a strong green light for a short while, lighting the area surrounding you.
You will have to navigate your way past the few obstacles (spiders, dropping platforms, moving platforms and many jumps) fast enough to avoid being caught in total darkness. At the end, you will find the power cell and an elevator that brings you back to the entrance.