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Ídhra (ē´ŧħrä) or Hydra (hī´drə), island, 21 sq mi (54 sq km), SE Greece, in the Aegean Sea, off the Argolís peninsula of the Peloponnesus. It is mostly barren and rocky. Ídhra town is the center of population. Sponge fishing, shipbuilding, textile manufacturing, and tourism are the main industries. Settled from the 15th cent. by Albanian-speaking Greeks from the Peloponnesus, the island became by the 17th cent. a shipbuilding and commercial center. It declined after the Greek War of Independence (1821–29), in which its seafaring people played an important role.

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