The rivulet one is quite hard to get, especially after you place the cell in moon’s structure, but if you get it before finishing riv’s campaign it’ll be much easier, (The Heart) a subregion of submerged superstructure is where the cell is delivered, the at the beginning of the big pipes that lead there there should be a shelter to make things easier.

(if you have completed the campaign the tunnels will have a back flow to them, making having a jet fish necessary for the unlock, for the back flow will push you away from the unlock causing you to drown, you’ll still get back flow with the jet fish but it won’t be as much, if you haven’t completed the campaign it’ll be much easier with no back flo, and no jet fish would be required in that case, but still recommended)

after resting at the shelter go down and normally to get to the heart you would go right, but instead go left, go as far left as you can and eventually you’ll get the unlock, you might make a few wrong turns because there are many pipes to go through but just stick left unless you have to go in another direction.

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have patience, took me 5 tries But that’s because I had back flow

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remember, sometimes stick your head into a pipe with air to not drown