Two rocky islands sit about 2.4 miles apart in the Bering Strait. Big Diomede belongs to Russia and Little Diomede to the United States, and the International Date Line runs between them, putting one 21 hours ahead of the other. This episode explains why the gap is 21 hours rather than 24, how the islands are the surviving mountaintops of the drowned Bering Land Bridge, and how they became the dividing line between two superpowers.

It follows Semyon Dezhnev's 1648 report of inhabited islands, Vitus Bering's 1728 sighting on the feast day of Saint Diomedes, Mikhail Gvozdev's 1732 geodetic mapping, the 1867 Alaska Purchase treaty that drew the boundary equidistant between the two rocks, and the Cold War Ice Curtain. It covers Lynne Cox's 1987 swim across the strait without a wetsuit, the Soviet removal of Big Diomede's indigenous population a ...