Enceladus was discovered on August 28, 1789, by William Herschel, but little was known about it until the two Voyager spacecraft, "Voyager 1" and "Voyager 2", passed nearby in 1980 and 1981. In 2005, the "Cassini" spacecraft started multiple close flybys of Enceladus, revealing its surface and environment in greater detail. In particular, "Cassini" discovered water-rich plumes venting from the south polar region. Cryovolcanoes near the south pole shoot geyser-like jets of water vapor, molecular hydrogen, other volatiles, and solid material, including sodium chloride crystals and ice particles, into space, totaling about per second. Over 100 geysers have been identified. Some of the water vapor falls back as "snow"; the rest escapes, and supplies most of the material making up Saturn's E ring. According to NASA scientists, the plumes are similar in composition to comets. In 2014, NASA reported that "Cassini" found evidence for a large south polar subsurface ocean of liquid water with a thickness of around .