The three Tharsis Montes volcanoes are evenly spaced about apart from peak to peak, in a line oriented from southwest to northeast. This alignment is unlikely to be coincidental. Several smaller volcanic centers northeast of the Tharsis Montes are on an extension of the line. The three volcanoes, most notably Arsia Mons, also all have collapse features and rifts, from which flank eruptions issued, that transect them along the same northeast-southwest trend. The line clearly represents a major structural feature of the planet, but its origin is uncertain.