In the cave to the west of the one with the dangling building and the glassed-in room with the 3 receptacles, shown in the previous section, are 2 boarded-up structures, piles of discarded tires, and other debris. The brown wooden crate is movable.
Pull it once away from the entrance. The go around to the side and pull it twice more...
...to position it under the opening in the ceiling.
Climb onto the crate and take a running jump to the roof of the building.
Drop through the broken tile (42/51) in the back corner. (This counts toward the "After Us the Deluge!" achievement/trophy.)
Follow the twisting tunnel...
...to an alcove with a switch.
Throw it to shift the boat back at the POOL toward the other ledge.
Continue to the end of the passage. Slide down the slope facing uphill and grab the edge of the opening.
Then drop down onto the crate you just moved. (If you didn't move the crate under the opening, you can still drop, but Lara will lose a little health.)
Now follow the mine cart tracks to the wooden door opposite the entrance. It opens automatically and closes again as you continue along the tracks into the tunnel beyond.
Striped barricades block the track and warn of boulder traps ahead. Advance to the first barricade to trip the boulder that rolls in from the left.
Turn right and grab the ledge that runs along above the track. Don't pull up yet.
Instead traverse to the left above the shallow pit where the tracks have collapsed. Now pull up.
As long as you don't go toward the boulder on the right, it will not roll. Turn left and follow the ledge.
Jump the gap and continue to the end.
Avoid the lava pit on the left and enter the tunnel on the right.
Here you'll find the FIRST FUSE (3) lying on the ground (along with another save crystal in NG+).
NOTE: If you accidentally trigger the boulder trap and manage to jump out of the way in time, you can still continue. Two different ways of doing this are described in a footnote below the main walkthrough.
DARK MODE: Even with brightness adjusted, dark images on a light background can be hard to see. May I suggest a reputable dark mode browser extension, such as Dark Reader? (Download only from trusted sites, like darkreader.org or your browser's web store.)
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