The Still Lands is the Relic Ruin that you will find in the desert area of Horizon: Forbidden West. I must say that I found this to be a rather frustrating relic ruin because I am not entirely sure the solution I found was the intended solution. It feels like there may be a bug or two in this puzzle. Either way, I was able to find a solution, which I would provide in this guide.
This puzzle does not require you to have a diving mask, but it does help to have it. Solving it can take quite a bit of effort with swimming. Ultimately, the main solution revolves around floating a crate across the water, which is part of where I feel the bug lies.
Solving the Stillsands Relic Ruin
The main solution for the puzzle at the Stillsands revolves around the water supply pump which can be found in a small side room. Once you turn it on, it will bring water into the room but it will not flood it. You will need to ensure a crate is pushed into the draining hole in the wall to trigger the room to flood.
It appears that it should have been possible for you to remove this crate. Aloy seems to suggest removing it but despite my best efforts, I could not figure out how to achieve this. If anyone has found out how, please post it in the comments below to help others out. Either way, it is possible to complete this relic ruin without needing to remove the create to bring the water level back down.
Once the entire building has been flooded, swim outside to the main room again and on the upper level, right next to the room with the water pump, you will find a second crate. Push this crate out and into the water below.
This begins the part of the quest that I felt was far too clunky to be the actual solution. You are likely supposed to remove the crate from the drain to lower the water level. Then you push the crate across the room to the other side where the relic is found. Then you raise the water level again so the crate is floating in the perfect spot.
Without being able to do this, you are going to have to rely on the grapple to pull the crate across the water to the other side. It is perfectly possible to do this and it didn’t take too much effort to get it across. Once the crate is floating in the right location, you will be able to climb onto it and then up onto the ledge, both of which are highlighted with yellow lines in the image above.