A large map on the wall inside of the old French post office in Ho Chi Minh City, formerly known as Saigon, reveals the original blueprint for one of the most spectacular river trips in the world: a cruise on the Mekong, which drains the former French colony of Indochina—now comprising Vietnam, Cambodia, and Laos.

The Mekong is the economic and spiritual spine of Southeast Asia as well as the aquatic highway to Angkor Wat, the 400-acre temple complex near Siem Reap, Cambodia. Built during the 12th century by King Suryavarman II of the Khmer Empire, it is the world’s largest religious structure.