isthmus, narrow neck of land connecting two larger land areas. Since it commands the only land route between two large areas and is on two seas, an isthmus has great strategical and commercial importance and is a favorable situation for a city. In modern times many isthmuses have been cut through by canals to eliminate the necessity of land transport. The most important isthmuses are the Isthmus of Panama, connecting Central and South America, and the Isthmus of Suez, joining Asia and Africa. Canals were dug through both of these. The Isthmus of Corinth between the Morea peninsula and central Greece also has a canal.

Tehuantepec, Isthmus of, c.125 mi (200 km) wide at its narrowest, S Mexico, between the Gulf of Campeche and the Gulf of Tehuantepec. It is mostly a rolling, tropical lowland with the lowest pass elevation at 754 ft (230 m) above sea level. Building of an interoceanic canal there was long considered, but estimated costs proved prohibitive. A transisthmian railroad between Coatzacoalcos and Salina Cruz was opened in 1907.

Perekop, Isthmus of, c.19 mi (30 km) long and from 5 to 14 mi (8-23 km) wide, S Ukraine, connecting the Crimea with the Ukrainian mainland. It separates the Gulf of Perekop (an arm of the Black Sea) in the west from the Sivash Sea (an inlet of the Sea of Azov) in the east. Because of its strategic position and economic importance (salt extraction from the lakes in the southern part), the Greeks and Tatars fortified the isthmus with moats and ramparts and the Tatars built a fortress on the site of the village of Perekop and called it Or-Kapi; there are ruins of the Greek and Tatar fortifications. The Greeks and Byzantines called the isthmus Taphros. Before the 15th cent. there was a Genoese colony there. The isthmus passed to Russia in 1783. There the Red Army decisively defeated (1920) Wrangel in the Russian civil war. In 1944 the Germans were routed out of the Crimea north of the isthmus. The isthmus was transferred with the Crimea to the Ukrainian SSR (now Ukraine) in 1954.

Kra, Isthmus of, narrow neck of the Malay Peninsula, c.40 mi (60 km) wide, SW Thailand, between the Bay of Bengal and the Gulf of Thailand. It has long been the proposed site of a ship canal that would bypass the congested Straits of Malacca.

Karelian Isthmus, land bridge, NW European Russia, connecting Russia and Finland. Situated between the Gulf of Finland in the west and Lake Ladoga in the east, it is 25 to 70 mi (40-113 km) wide and 90 mi (145 km) long. Saint Petersburg and Vyborg (Viipuri) are its chief cities. Originally part of the Grand Duchy of Sweden, the isthmus passed to Russia in 1721, and—except for its southernmost section—became part of Finland in 1917. The Mannerheim Line, which crossed the isthmus, was breached in 1940 by the Russians, who occupied the area. It was briefly held (1941-44) by Finnish and German units during World War II. The isthmus was formally ceded to the USSR in 1944, and more than 400,000 of its Finnish residents moved into Finland.

Corinth, Isthmus of, c.20 mi (32 km) long and 4-8 mi (6.4-12.9 km) wide, connecting central Greece (Attica and Boeotia) with the Peloponnesus, between the Gulf of Corinth and the Saronic Gulf. It is crossed by the Corinth Canal, built between 1881 and 1893, which connects the Aegean and the Adriatic seas. Parallel to the canal are ruins of the ancient Isthmian Wall, which was restored (3d-6th cent. A.D.) by Byzantine emperors to defend the Peloponnesus. Near the eastern end of the wall are ruins of the sanctuary of Poseidon where the Isthmian games were played.