There are two islands just a mere 2.4 miles (3.8 km) apart in the middle of the Bering Strait between Alaska and Siberia, but going from one to the other would put you almost a full day ahead or behind, depending on which Island you were going to.

The two islands in question are the Diomede Islands, which are separated by the International Date Line, the marker when one calendar day begins and the other ends. One island on the Russian side, called Big Diomede, sits 2.4 miles (3.8 km) away from the other island, Little Diomede, on the Alaskan, United States side.

Big Diomede is technically 21 hours (20 hours in the summer) ahead of Little Diomede despite the short distance apart. In winter, an ice bridge forms between the islands, making it technically possible to walk between them. In the summer, someone could kayak or sail between them. It’s illegal to go from one island to the other, however. Big Diomede is sometimes referred to as “Tomorrow Island,” and Little Diomede is referred to as “Yesterday Island” because of their vast time difference.

Other Facts About the Diomede Islands

Little Diomede is inhabited by about 115 people, according to the US 2010 Census. Big Diomede doesn’t have a permanent settlement but has a Russian weather station.

The islands were discovered by Danish explorer Vitus Bering (who the Bering Strait is named for) on August 16, 1728, on St. Diomede’s Day, a day celebrated by the Russian Orthodox Church for St. Diomede.

Big Diomede was the site of a Russian military base in World War II, and any residents were moved to mainland Russia during this time.

In 1987, American swimmer Lynne Cox swam between the islands from Little Diomede to Big Diomede. She started lobbying the Soviet government in 1976 to swim across the border, and approval came just 24 hours before the swim was scheduled to take place when President Mikhail Gorbachev gave permission. The swim took two hours and five minutes in 38 degrees Fahrenheit (3 degrees Celsius) water and was meant as a gesture of peace between the two countries during the Cold War.

The inhabitants of Little Diomede get their mail weekly by helicopter, though ski planes occasionally land there in the winter on an ice runway. The majority of supplies arrive by barge once a year.

Sources: PBS, Britannica, Mirror UK, Alaska Centers, Earth Observatory, NPS