The Hidden City – Returning to Unuratu's Prison Later in the Game

This document is inside Unuratu's prison cell on the far side of the gorge east of the city. If you missed it during the 'Rebellion Lives' story mission, when you rescued the queen, fast travel to the Temple of Kukulkan Base Camp, which appears on your map after you've cleared this area and moved on to Porvenir Oil Fields.

Cross the bridge to the island in the middle of the gorge.

Climb the stone steps near monolith #7.

Then, instead of crossing the second bridge, head for the southeast corner of the island.

Grab the rope line and climb across the river.

Drop down off the rope and squeeze through the narrow opening on the right to re-enter the prison cell.

The document is sitting on a shelf near the burning brazier.