Khongoryn Els is a wall of sand a hundred kilometres long in the Mongolian Gobi, and it makes a noise. When a dry slope slides, the sand lets out a low, carrying hum, and that is what the Mongolian name means: Duut Mankhan, the singing dunes. Very few dunes anywhere can do it. The recipe is fussy — rounded, silica-rich grains in a narrow size range, stacked deep, and bone dry — and most sand fails at least one of those conditions. Rain silences these ones at once. How the slide turns into a single deep note is still unsettled. One camp holds that the loose surface layer works as a waveguide whose depth sets the pitch; another that chains of grains resonate inside a band only centimetres thick.
At the foot of all that sand runs a thread of water. The Khongoryn Gol, fed by underground flow off the mountains, keeps a ribbon of green meadow alive against a hundred kilometres of desert, and you can stand with one boot on turf and the other on sand. Waders work the stream — curlew, snipe and redshank, birds with no business in a desert — and demoiselle cranes and ruddy shelduck drop in on passage. Bactrian camels plod the strip at a pace that rocks you side to side, and the wall grows absurdly larger the whole way in. They are not props: a herding family's camels are its savings, and the calves come in March and April, at the cold tail of the Gobi winter. Herders at the dune's edge put travellers up in felt gers heated by a dung stove. Late in the day the light comes in almost sideways, every crest divides into a lit face and a black one, and the colour runs from pale gold to copper. Then you grind up to a high crest, which costs an hour of sliding back with every step, sit down on the steep face and push off. The slope goes in a sheet underneath you and the hum comes up through your chest — and it stops the instant the sand does.