Sentence:  Diamonds were driven to the earth's surface by a very unusual, super-long volcanic shaft, often called a <WSD>pipe</WSD>. While the <WSD>pipe</WSD> of a conventional volcano may extend down 50 miles or so, the volcanic <WSD>pipes</WSD> that pick up diamonds along the way had to go much deeper, perhaps as deep as 300 miles. ... These <WSD>pipes</WSD> thrust like sharp swords, delivering piles of precious gems by penetrating the tough hide of mother earth. They exploded to the surface; for a typical diamond, the journey through the <WSD>pipes</WSD> took just hours. No human has ever seen a diamond eruption. 
Meaning of the word marked with <WSD> tags:  A vertical conduit through the Earth's crust below a volcano through which magma has passed, often filled with volcanic breccia. (from 19th c.)