Only one thing is clear about why knuckles crack, pop, and crunch: there are gas bubbles involved.
Whether the noise comes from the birth or burst of those bubbles has been a source of simmering debate for decades, though. Even with modern techniques and studies, new evidence still hasn’t resolved the issue completely. But the latest data is least intriguing—it shows that there’s a mysterious blast in the knuckle after the popping noise. Take a look: