Pack mules carrying United States Postal Service mail bins traverse the trail between the Hualapai Hilltop and the Supai Village on June 21, 2011, on the Havasupai Indian Reservation in the Grand Canyon. (Photo by Julia Beverly/Getty Images)Julia Beverly/Getty Images

United States — a post office where mules deliver the male

At the bottom of the Grand Canyon, the Supai Post Office receives mail via mule, continuing a tradition that dates back to the 1930s. Six days a week, 10 to 22 mules carry mail, food, and supplies down a 9-mile trail, taking roughly 3 hours downhill and 5 hours back up. Each mule carries up to nearly 200 pounds, divided and balanced evenly on each side. On the return, wranglers untether the mules, which navigate the trail independently. The post office, located in Supai Village, issues a special Mule Train postmark and is the last official mail-by-mule route in the country—and likely one of the last in the world.

Slovenia — a post office in a cave

Postojna Cave in southwestern Slovenia stretches 15 miles and is over two million years old, making it Europe’s most visited cave system. Among its stalactite-lined cavernous galleries, an underground post office has operated since 1899, originally in a small stone building in the Dance Hall. By 1927, it moved to the larger Concert Hall, processing over 90,000 letters annually. Today, it remains the world’s only post office in a cave. Visitors can post mail surrounded by towering stalagmites and crystalline formations. The cave itself includes a railway line and a concert hall, but the post office stands out, blending practical service with a surreal subterranean setting that has welcomed more than 40 million tourists over two centuries.