Definition of "rille" [rille]

• A long, narrow depression in that resembles a channel, found on the surface of various lunar and planetary bodies. (noun)

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• "The remnants of volcanic tubes on the moon whose roofs have capsized and a trench or valley is created is called a rille system, which is a groove or long narrow depression on the lunar surface."
• "It sits in the middle of a rille, suggesting the hole leads into a lava tube as wide as 370 metres across … Since the tubes may be hundreds of metres wide, they could provide plenty of space for an underground lunar outpost."
• "Two big ones and six little ones; how difficult could it be to find the rille?"