Why Panama is an S

A land bridge

Panama occupies the Isthmus of Panama, the narrow bridge of land that joins North and South America.^[1] For millions of years an ocean flowed between the two continents. Smithsonian scientists and colleagues, weighing fossils, rocks and the genes of sea animals split by the isthmus, dated the final closure to about 2.8 million years ago, when the Pacific and Caribbean stopped mixing.^[2]

The land that rose is not straight. Panama has an elongated S shape: its Caribbean coast runs about 800 miles (1,290 km) and its Pacific coast about 1,060 miles (1,700 km), yet a line from the Costa Rican border to the Colombian border is only 480 miles (770 km). At its narrowest, from the Gulf of San Blas to the mouth of the Chepo River, it is about 30 miles (50 km) across.^[1] A spine of mountains runs almost its whole length, broken near the middle by a saddle of low ground, which is where the canal was dug.^[1]

The canal that runs south

Because the isthmus bends, the Panama Canal does not cross it from east to west, as most people suppose. From Colón on the Atlantic side it runs due south to Gatún Lake, then turns east and heads south-east to the Bay of Panama. Its Pacific end near Balboa is about 25 miles (40 km) east of its Atlantic end.^[4]

A country cut in two

Panama was part of Colombia until 3 November 1903, when a revolutionary junta declared independence after Colombia rejected a canal treaty with the United States; US ships helped keep Colombian troops away.^[3] Fifteen days later the Hay–Bunau-Varilla Treaty gave the United States, in perpetuity, the use and control of the Canal Zone, a strip 10 miles (16 km) wide across the middle of the new country. The canal opened on 15 August 1914.^[3]

For most of the century the zone split Panama in two. Treaties signed by Jimmy Carter and Omar Torrijos in 1977 provided for its gradual return; they took effect on 1 October 1979, and on 31 December 1999 Panama took full control of the canal.^[5]