Far Cry 6 is filled with missions that send you searching for items that help with another mission.
As Dani Rojas, you will eventually come across Juan Cortez in Far Cry 6. On one of his quests, he will task you with stealing what he calls "a thing". That thing is a cache of uranium found in Fort Quito.
The trick here, though, is that Fort Quito is locked. To gain access to this Far Cry 6 fort, you will have to go on the hunt for its key. More than likely you'll come across the door first after clearing the fort of enemies and will have to back track.
Far Cry 6: Where to locate the Fort Quito key
There will not be a marker or any indication of where the Fort Quito key is in Far Cry 6 until you are right next to it. So, you will need to know precisely where to locate it if you want to unlock the armory and retrieve the uranium.
The key is found inside of a briefing room. Once you are at the locked armory door, turn directly around and go up the stairs. There is an open doorway at the top that you can enter.
Don't walk too far into the room once you go through the doorway. Instead, take an immediate right to find a long table. The table is covered with papers, notebooks, a coffee mug, and a lamp.
Far Cry 6 will give you a HUD notification showing that you have located the Fort Quito armory key as you approach the table. Pick it up at the far end of the table, and you can head back down the stairs.
Use the key to open the Fort Quito armory door and right inside will be the massive cache of depleted uranium that Juan Cortez wants. Bring the uranium back to Juan.
Once you've returned with the "thing", Juan Cortez will show you how to make some of the most dangerous weapons in all of Far Cry 6. These rewards make this mission more than worth the effort.