I could tell you where it is, but that would spoil all the fun...
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A particle pole.
A particle bridge spanning a river of magma.
The particle structures are Magical structures consisting of fast moving red and orange specks that seem intangible but actually behave as solid structures. They come in two forms: a platform strong enough to support people and a pole that can be climbed in a similar method as a rope. Both types occur only in The Maw of Chaos during Thief: The Dark Project/Thief Gold.[1]