Astacus // (Greek Ἀστακός Βιθυνίας) is an ancient city in Bithynia; it was also called Olbia //. Stephanus of Byzantium records an aetiological myth that it was founded by Astacus, son of Poseidon and the nymph Olbia. The city was founded in the Second Greek colonisation by the Megarans together with the Athenians.
The traditional date of the founding is 712/11 BC, the first year of the 17th Olympiad. However, "Diodorus Siculus" (aka "Library of History"), Book XII, Chapter 34, writes that in the year 435 BCE "And while these events were taking place [the battle of the Athenians on the isthmus near Pallenê against the Potidaeans] the Athenians founded in the Propontis a city which was given the name of Astacus." (Perhaps Diodorus was incorrect.)
King Zipoetes I of Bithynia made two attempts to absorb Astacus into his kingdom: in 315 BC he was defeated by succors sent by Antigonus Monophthalmos. In 301 BC, he was successful, but the city was destroyed in the war.
Nicomedes I, son of Zipoetes, founded a new city to replace Astacus across from its former location, which he named Nicomedia after himself, bringing some of the Astacan cults to the new site. Nicomedia remained the capital of Bithynia, and became one of the great cities of the Roman east; the Emperor Diocletian made it his usual capital.
Ἦν δ' ἐν αὐτῷ τῷ κόλπῳ καὶ Ἀστακὸς πόλις, Μεγαρέων κτίσμα καὶ Ἀθηναίων καὶ μετὰ ταῦτα Δοιδαλσοῦ, ἀφ' ἧς καὶ ὁ κόλπος ὠνομάσθη: κατεσκάφη δ' ὑπὸ Λυσιμάχου: τοὺς δ' οἰκήτορας μετήγαγεν εἰς Νικομήδειαν ὁ κτίσας αὐτήν. (And on the gulf itself there was also a city Astacus, founded by the Megarians and Athenians and afterwards by Doedalsus; and it was after the city Astacus that the gulf was named. It was razed to the ground by Lysimachus, and its inhabitants were transferred to Nicomedeia by the founder of the latter.)