"One of the most bizarre features yet identified on Venus is a remarkably long and narrow channel that [the spacecraft] Magellan scientists have nicknamed the River Styx. Although it is only half a mile wide, Styx is 4,800 miles long. What could have carved such a channel in unclear. Water, of course, is out of the question. Flowing lava is a possibility, but it would have to have been extremely hot, thin, and fluid."
Another suggested fluid is sulphur, but there is still room for speculating about exotic fluids, given Venus's high surface temperatures. Another point of interest: the River Styx does not run steadily downhill. It takes an up-anddown course. Either the Venusian topography has shifted since the Styx was cut, or the channel is not a river at all but rather some bizarre geological feature.
(Chaikin, Andrew; "Magellan Pierces the Venusian Veil," Discover, 13:22, January 1992.)
"Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada>, Vol. 81, no. 1 (1987), p. 24., 02/1987
