Ancient Cistern refer to special collectible Documents found in Rise of the Tomb Raider.
There are two total Ancient Cistern documents in the game.
Correspondence[]
My Love,
This letter is long overdue, but I have been hard at work. When you arrive, there will be a home for you here, with all the comforts we have left behind. It is a huge task ahead of us, to design and supervise the construction necessary to provide fresh water to our people, but all my labor will be paid in full when I see you and our children's faces again.
We will create a new world here, the equal of everything you left behind to follow me. It will not have been in vain, as long as you and I are together.
Lara's Thoughts[]
A letter from the foreman to his wife... He expected her to join him at any moment.
A Warning[]
This should be more than enough oil to light the passages below, but be careful transporting it. We have not perfected the refining process, and it is still volatile,
You will know the man who supervises the construction, we all do. without his knowledge of the Earth, none of us would have survived along the difficult journey. Do not linger, and if he asks for word of his wife and child, feign ignorance.
He cannot accept that they perished during our exodus, and expects them to arrive any day. We need his genius, and I fear if he saw the truth, we might lose him entirely.
Lara's Thoughts[]
Instructions to deliver oil to the workers below... And not to upset the foreman.