The sand on these beaches makes mysterious musical sounds when stepped on

The "singing" or musical sound is actually a high-pitched or squeaking noise that resembles the sound of a musical instrument or a whistling wind. 

The exact cause of this is not fully understood, but researchers have come up with several theories to explain why this happens.

One theory suggests that the size and shape of the sand grains can cause this because, when the grains of sand are of a particular size and shape, they rub against each other as they shift under pressure, such as when someone walks on the sand. The rubbing action creates vibrations that produce a musical sound.

Another contributing factor is the moisture content of the sand. The sand must have a specific moisture content to create the sound and when the sand is at the right amount of moisture, it can vibrate to produce the musical tones. 

Environmental conditions, including temperature and humidity, are also some of the causes. Singing sand is more likely to produce sound on warm, dry days when the sand's moisture level is just right.

Apparently, not all sandy beaches can produce these sounds, and it remains a somewhat rare and mysterious occurrence. 

Here are some well-known locations where singing sand has been reported: 

Whistling Sands, Wales, United Kingdom

Whistling Sands, also known as Porthor Beach, is located on the Llŷn Peninsula in Wales. The sand at this beach is popularly known for producing high-pitched, musical sounds when you walk on it, which is how it earned its name "Whistling Sands."

Singing Beach, Manchester-by-the-Sea, Massachusetts, USA

Singing Beach in Massachusetts is another well-known coastal beach where the sand produces musical sounds when walked upon. The composition of the sand grains makes it one of the few coastal locations where this happens.

Kelso Dunes, California, USA 

Although not a beach, the Kelso Dunes are located within the Mojave National Preserve in California. These dunes, situated within the larger Mojave Desert, are also known for the sounds they produce when people walk on them. 

Researchers may have made some progress in understanding the factors that contribute to the singing sand phenomenon. However, it's still an area of ongoing study as this mystery has not been fully solved yet.