The Coolest Places Around the World From Where You Can Mail a Letter | Condé Nast Traveler

"I observed that mail bound for Supai Village at the bottom of the Grand Canyon is still delivered by mule: six days a week, teams of 10 to 22 mules carry mail, food and supplies down a 9‑mile trail (about three hours downhill and five hours back up), with each mule carrying up to nearly 200 pounds balanced on each side. The return sees wranglers untether the mules so they can navigate the trail independently; the post office issues a special Mule Train postmark and remains the last official mail‑by‑mule route in the United States." - Zara Flavia Dmello