As this illuminating history of the Martian canals controversy notes, when sky gazers began examining the planets through telescopes in the seventeenth century, Mars was a relatively featureless disc compared to the visual dramas of Jupiter, Venus, and Saturn. However, as telescopes improved, investigators began to make out various shapes on the surface, which, for years, many claimed were perhaps constructed channels or canals. This excellent site compiled by Dr. Barbara J. Becker of the University of California, Irvine details the development of the idea with drawings, maps, photographs, and a detailed history. Readers may note that this lecture is number 19 of 20 in Becker's course, Exploring the Cosmos; similar lesson plans for the remaining lectures may be found at the bottom of the page.