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). Take a standing jump (no need to run) to the small, floating platform that's third from the left in the second row of platforms. Walk to the front edge of the small platform and jump to the stone ledge.
When you step to the far side of this ledge, three new platforms rise up from below, but you can ignore those. Instead, step back so Lara can get a decent run-up. Then take a running jump over the low wall where it angles down in a V shape to land in the tunnel opening ahead. (Thanks to Luke for this tip.)
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