Absheron (əpshĬrôn´) (äpshĬrôn´) or Apsheron (əp–), peninsula, c.40 mi (60 km) long, extending into the Caspian Sea, E Azerbaijan. It is a dry, hilly area at the eastern end of the Greater Caucasus Mts. and is underlain by rich oil-bearing rock strata. The oil industry developed there in the 1870s, although the presence of oil was known long before. The peninsula, with its Bakı (Baku) oil fields, was the USSR's chief oil-producing region prior to the opening of the Siberian fields. Natural gas wells, salt lakes, mineral springs, and mud volcanoes are also found on Absheron. Bakı is the region's chief city, and the peninsula falls within the boundaries of Greater Bakı.