Tethys was both sister and wife of Oceanus, the earth-encircling, fresh-water stream. By him she was the mother of the Potamoi (Rivers), Oceanids (Springs, Streams & Fountains) and Nephelai (Clouds). Tethys was imagined feeding her children's streams by drawing water from Okeanos through subterranean aquifers. Her name was derived from the Greek word têthê, "the nurse" or "grandmother." She is the mother of Amphitrite, and therefore Poseidon's mother-in-law.