FOR SOME 3,500 YEARS, the sand dunes of Turkmenistan’s Murghab Delta plains, a stretch of Central Asia’s Karakum Desert, concealed the ruins of a sprawling 70-acre city belonging to a long-forgotten civilization, its name lost to history. First rediscovered by Soviet archaeologists in the 1970s, the city was dubbed Gonur Depe, or “Gray Hill,” which is what the local Turkmen called the site. The sandy mound concealed high mudbrick walls, carefully gridded streets that divided residential districts from those where merchants may have plied their trades, and areas where religious ceremonies likely took place. Soon after the city’s discovery, excavations revealed that the people of Gonur Depe were immensely wealthy. Artisans crafted beads from lapis lazuli imported from Afghanistan, carved ivory from India into mosaics depicting dragon-like creatures, and fashioned…