National Geographic : 1972 Feb
JAMESB. IRWINANDDAVIDR. SCOTT(BELOW) surface. Scientists offer several explanations for the mysterious canyon that wanders through this waterless, windless environment: Flowing volcanic ma terials may have cut the gorge; gases erupting through cracks could have left the 70-mile-long fissure; by a third theory, the rille may have opened when the cooling Imbrium lava shrank. Photographed with a telephoto lens, the rille in the view below appears much narrower than its actual one-mile width.