in the solar system. Leading theories include lava channels, collapsed lava tubes, near-surface dike intrusion, nuée ardente (pyroclastic cloud), subsidence of lava-covered basin and crater floors, and tectonic extension. On-site examination would be necessary to clarify exact methods. Sinuous rilles According to NASA, the origin of lunar sinuous rilles remains controversial. The Hadley Rille is a 1.5 km wide and over 300 m deep sinuous rille. It is thought to be a giant conduit that carried lava from an eruptive vent far to the south. Topographic information obtained from the Apollo 15 photographs supports this possibility; however, many puzzles about the rille remain. See also Arachnoid (astrogeology) Chaos terrain Dark dune spot List of features on the Moon Lunar lava tube Martian spiders (dark dune spots) Planetary geology References General Ewen A. Whitaker, Mapping and Naming the Moon, Cambridge University Press, 1999, . American Astronomers Report: What Formed the Moon's Sinuous Rilles?, Sky & Telescope, Vol. XXVI, No. 1, July, 1963. Atlas of Lunar Sinuous Rilles Geological features on the Moon Valleys and canyons on Mars Planetary geology Volcanic landforms Volcanism on the Moon