The name of a
Titan in ancient
Greek mythology, a child of
Uranus and
Gaia, married to her brother,
Ocean. The mother of 3000 ocean
nymphs and all the river
gods.
(Thank you, mythopedia.hypermart.net.)
Not to be confused with Thetis, a Nereid, wife of the sea god Poseidon.
Also, the name of an ocean that stretched from present-day Southern Europe to South-East Asia 200 million years ago.
Plate tectonics (in particular, the northward movement of Africa, then Gondwanaland, against the Eurasian plates, then Laurasia), gradually narrowed it to a narrow band, running west-east; already closed off at the West end, the collision of the India plate onto the Asian continent, 50 million years ago, also closed it off from the Indian Ocean to form an inner sea that dried up to leave the present-day Aral Lake, Caspian Sea, Black Sea and Mediterranean as remains. The Mediterrean and Black Sea were reconnected to the oceans very recently (the opening of the Bosporus may well be attested in surviving legends); the other remains are being used up at record speed by irrigation.