Back away from the edge so Lara is standing at the top of the slight slope in the tunnel opening (where there's a seam between the textures). Jump to the floating platform that's third from the left in the second row of platforms. When you land here, some of the platforms ahead fall away, others arise.
Activate walk mode. Turn right. Step to the back of the platform and take a short hop to the next platform. (This jump is illustrated by the second arrow in the screenshot above.)
Again, step to the back of the platform and take a short hop to the next platform, just below the alcove. When you land there a new platform appears overhead.
Turn left, walk to the edge of the platform and jump straight up to grab the one above. Pull up.
Turn right and jump into the alcove. Pick up the Aqua-Lung if you want it.
To get down, postion Lara in the opening, adjust her angle and jump down onto the larger stone ledge below.
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