People once widely believed that intelligent life existed on Mars. The 19th-century discovery of what appeared to be geometric designs cut across the surface was taken as evidence. The lines were thought to have been a system of canals that had been built to irrigate the surface. It is now clear that "canals"—perhaps the most spectacular geologic features of Mars—are natural valleys where ancient rivers once flowed. Another fragmented idea concerns the planet"s seasonal changes in color. Once attributed to the rapid spread of some life-form, these shifts are now known to develop from the movement of fine dust in the atmosphere. By the close of the 20th century none of the many experiments conducted by spacecraft had ever found persuasive evidence of life. Nevertheless, speculation continued over the existence of some form of life, in either the present or the past. In 1996 scientists discovered organic compounds and minerals in a meteorite(陨石), consisting of Martian rock, that collided with Earth around 11,000 B.C. These compounds suggest that Mars may have been inhabited by organisms more than three billion years ago.