[dunes rumbling] - [Joe] That's not the sound of the drone you're hearing. It's the dune. Actually, can you turn that up a bit? [dunes rumbling] We're in Dumont Dunes, California, one of 40 or so places in the world where dunes sing. They're called booming sand dunes. Kind of mysterious, right? The truth is, we still have a lot more to learn about dunes and sand, in general. And those mysteries have very important implications for all of us. - [Nathalie] A sand dune is a hump of sand at first sight. But actually, if you look within it, there's so much more, and it's really a mystery. - [Joe] That's Dr. Nathalie Vriend, one of the original crew who first solved the mystery of the booming dunes. She still studies dunes, but from her lab in Cambridge. - [Nathalie] So if you think about sand, you can hold it in your hands. As you pour it out of your hand, it acts as a fluid because it flows very easily, and it falls on the ground and it forms a heap that is more like a solid. And now, if you throw it up in the air, it actually blows away by the wind, so then it acts as a gas. So sand is a very interesting substance because sometimes it behaves as a fluid. Sometimes it behaves as a solid. Sometimes it is a gas. - Granular flow is a pretty poorly understood field of physics. Wait, wait, I talked a bit about that for an episode of "It's Okay To Be Smart". Let's cut in some of that. Okay here you go, Joe. Thanks, Joe. You see these ripples? They form just about anywhere sand is moved by wind. First, the sand grains are lifted into the wind. When they land, they splash into other grains. If a tiny pile randomly forms, it catches more of this bouncing sand. Behind that hump, a shadow forms where fewer grains land. A little ways down, sand starts splashing and piling up again. This feedback loop continues on and on to create those neatly organized ripples of sand, and on the larger scale, wind, sand and physics combine to create dunes. When wind encounters a large bump, it's deflected up, compressed, and it speeds up, similar to air moving over an airplane's wing. This accelerated wind erodes the sand on the upstream face of the dune and deposits it near the top. But sand can only be stacked so high. When the downstream face of a sand pile reaches 33 or 34 degrees, known as the angle of repose, it spontaneously avalanches. Sand avalanches have everything to do with that booming we heard about earlier. - [Melany] How can a sand dune generate a musical tone? How can nature give us something that sounds like a cello? - [Joe] We're with Caltech professor of Mechanical Engineering, Dr. Melany Hunt. - [Melany] This booming dune problem is something that people have thought about for centuries. - [Joe] Ancient texts from around the world, some over a thousand years old, guessed at the source of the booms. Charles Darwin heard it in the Chilean desert. There are accounts of booming in the Sahara. Even Marco Polo apparently heard it. - [Melany] When you take someone for the first time, they are very surprised by what they hear. People would look around and think that there must be something else there, a giant worm coming out of the sand. - [Joe] To be clear, a giant worm rising from the earth is not totally out of the question. That's "Dune" the movie for anyone who's had their head in the sand for the last year or 60. Okay, no, actually turns out we can now explain this with science. Dr. Hunt, her colleague, Rob Clayton, and her former PhD student, Nathalie Vriend, remember her from earlier? they took dozens of trips to the desert to study the booming dunes. They had a grasp of some of the factors that enabled the booming to occur but uncovered exactly what was going on, structurally, to create the sounds that people had been hearing for generations. In order for dune booming to be heard, the conditions have to be just right. - [Melany] The sand has to be really, really dry. - [Joe] Like summer in the desert dry, and summer at Dumont Dunes is no joke. It was a sweltering 118 degrees Fahrenheit when we first showed up in the late afternoon. The dune has to be big, like over 120 feet tall. That's because, for the dune to boom, the dune has to have the correct internal structure. - [Melany] There is this upper layer of sand that is maybe about a meter or a meter-and-a-half in depth. The sound doesn't travel very well through that, but that's where that frequency comes from. It's that thickness of this loose layer that lies above this hard reflector. - [Joe] An additional component seems to be that the grains themselves have to be the right size and shape, and the wind acts like a size separator. - [Melany] The big sand grains never make it to the top of the dune. They're too large to be carried by the wind. And small, fine sand has been lifted off and transported away. So what you find is that, at the top of the dune, it's just much better sorted sand. - [Joe] To understand this, let's think about a cello for a second. The bow of a cello provides friction energy, which is then converted into vibration energy by the string, and certain frequencies of vibration are amplified by the wood of the body interacting with the air and the size of the cello's cavity. The sand avalanche provides energy, just like the bow. The moving sand grains put vibrations into the air, like the wood of the cello, and the thickness of the sand layer determines which resonant frequencies are reflected between the air and the non-moving sand layer below, just like the cavity of the cello. To begin this process, Dr. Hunt and her students scoot down the dune in a line. - [Melany] Where you first start, you really hear nothing, and then there's this low rumble that develops, and the rumble can continue for 10 seconds, but it can continue up to a minute or so. [people scraping sand] [sand dune rumbling] It's really quite remarkable what it does. - [Joe] The waves actually move individual grains of sand and exert force in the same way that sound can move molecules of air. - [Melany] It's this frequency from about 75 hertz up to about 100 hertz. - [Joe] Those frequencies can be tied to musical notes. [note humming] What you're hearing now is closest to an E-flat, but it's sometimes an F-sharp or a G. [notes ding] - [Nathalie] But this is a natural phenomenon that people don't expect. It's a mystery, and there's this natural curiosity to explain the unexplained. With my booming sand dunes research in my PhD, I was able to explain the mystery, but the sand dunes have so many more unanswered questions. So from there, I went on a quest to identify still open questions involving dunes. It is essential that we understand how dunes are formed, how they evolve and how they move. 20 years ago, a study was done, and they found that an area of 120,000 square kilometers of agricultural land was lost to desertification every year. To put this in perspective, this is the size of the state of Mississippi every year. This is due to dune encroachment and desertification. Because of climate change, this only became worse in the last few decades. - [Joe] The European Commission's "World Atlas of Desertification" reported that 75% of Earth's surface had already been degraded, and that figure could jump to 90% by 2050. Areas like the Sahel, the Gobi Desert and parts of South America are experiencing disproportionate effects of desertification. - [Nathalie] And these are actual people that have their livelihoods impacted. - [Joe] The people that live in these so-called dry lands face food insecurity due to loss of arable land and have to deal with dangerous dust storms that can damage their lungs. - [Nathalie] I would really like to contribute a small part in understanding how we can reduce the mobilization of these dunes and perhaps divert the path of dunes specific and protect the people downstream. In the last few years, I've done some exciting work with my PhD student Karol Basick in the lab on sand dune migrations. These large desert dunes, they take years to move. So in the laboratory, we're using an overlaying fluid instead of air, and because we're using fluid, we can see these processes in a matter of minutes, instead of years. - [Joe] What Nathalie and Karol found in using their experimental dune racetrack was surprising. The miniature dunes communicated with each other, negotiating for space. When they put two dunes of the same size in the tank, initially the front dune moved faster than the back dune. But after a period of time, the front dune slowed down until they were moving at about the same speed and eventually settled at a distance of 180 degrees apart from one another on opposite sides of the tank. - [Nathalie] The interesting question, however, is how does this relate to the real world? Do we observe the same behavior in large dune fields, and subsequently whatever we observe from satellite images in large dune fields, can we actually mimic it in the lab? That's what I'm hoping to continue to research in the next few years. The fundamental work that we're doing is essential to add to our knowledge base. It's just that we can protect livelihoods of people and infrastructure downwind of these marching dunes. - Through the most simplified lens, these dunes may be nothing more than sand, wind and time made tangible, but although they may be merely a product of physics and geology, they are more than that, too, something more than dead that evolves and even moves, a thing, though not quite alive, that can still sing. [ethereal music]