Sentence:  ( 1 ) As far down as 400 <WSD>miles</WSD> below the surface the material should be hot enough to be plastic and adjust itself to twisting forces by sluggish flow rather than by breaking , as rigid surface rocks do . 
Meaning of the word marked with <WSD> tags:  a unit of length equal to 1,760 yards or 5,280 feet; exactly 1609.344 meters