Hidden One Bureaus

Take one of the oil jars that stand slightly north of the marker and climb up the fallen tree trunk.

Use the jar to blow up the entrance to the underground.

Keep going through the tunnels and when you reach the place shown in the picture below, climb up.

At some point you will have to move the rack blocking the passage and eventually you will reach a large flooded room.

Get to the other side and climb up to the opening on the left.

Dive in and find the key.

Get out of the water and use the key to open the padlocked door nearby. You can now shoot the lock on the door on the other side.

Get to the other side and enter through the door you just unlocked to the room with the gear chest and the key that will allow you to open the door to the room with the ingot chest (you have to swim through the underwater tunnel to get there).

Destroy the boards blocking the entrance and jump down.

Swim through the underwater tunnel until you can exit the water and enter a large room. Go to the right side and move the rack behind which there is a small passage near the floor.

Keep walking forward and after a while you will reach your destination.

To get inside you must first make a leap of faith.

Then dive, destroy the wooden wall and swim through the flooded tunnel.

Jump on the platform, then on one of the ropes and shoot the oil jar.

The explosion will create a hole through which you can enter the bureau.

To get to the room where the chest is located, shoot the pulley on which a net with rocks is attached.

Falling stones will destroy the blocked passage and you will be able to get inside.

To get to the bureau go down to the basement of the Roman ruins.

Destroy some crates and go through the hole hidden behind them near the floor.

Go through the gas-filled tunnel. You can use a torch to get rid of it for a while.

Destroy the boards in the floor and jump down. You will reach a large room filled with gas.

Burn off the gas and go to the other side.

In the next room you will find jars of oil...

...which you can use to blow up another stone wall at the end of the corridor (gas-filled and dark).

When you blow up the wall you will be able to enter the bureau.

Jump down into the "large well".

Dive in and swim through the underwater tunnel.

Move the rack and unlock the door through which you will return to the well.

Grab a jar of oil

Use the jar to destroy the stone wall in the side tunnel.

The main area of the bureau is on the other side.