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:

The video is from a new study from a team led by Robert Boutin, professor of radiology at the University of California, Davis. The researchers used ultrasound imaging to watch 400 knuckle pulls, 62 of which produced audible cracks. The bright bursts, like the one seen in the ultrasound above, corresponded to the audible cracks 94 percent of the time. But based on the timing, the source of the flash is not the source of the noise. Boutin told Ars that he doesn’t think the flash is a bubble pop either. “Although the bright flash appears suddenly, the bright—hyperechoic—area lingers,” he explained.

The finding was presented this week at the annual meeting of the Radiological Society of North America in Chicago, and it adds to the mystery and controversy of what exactly goes on when joints crack.

The debate started with a 1947 study in which researchers used serial radiographs to conclude that formation of gas bubbles in knuckle joints—or metacarpophalangeal (MCP) joints—are what cause the bodily blasts. The researchers found that the noise occurred as the MCP joint was pulled apart, and researchers speculated this decreased the pressure in the joint. This decreased pressure might allow gases previously dissolved in joint fluid to pop out of solution and form a bubble. The researchers reported that initial appearance of a clear bubble space in their joint images coincided with the cracking sounds. And the researchers further linked bubble formation—and cracking—to normal, healthy joints.

But in 1971, another group used similar techniques to come to a different conclusion: the crack was from the bubble bursting. This led some researchers to believe that such knuckle-cracking explosions could also damage the joint.

Ever since, knuckle cracking has been a bone of contention among scientists. Some have argued for and against the two camps—bubble birthers or bursters—and others have come up with entirely different explanations, such as ligament recoil.

In April, a Canadian research group published a study that seemed to support the bubble formation argument, but the authors’ explanation was slightly different from the one from 1947. That study, led by Gregory Kawchuk of the University of Alberta in Canada, used magnetic resonance imaging (MRI) data of one man’s knuckle’s cracking. Like the 1947 study, the images show that the popping sound coincided with gas bubble formation. However, the researchers believe the bubble forms not from dissolved gas crashing out of solution, but from a process called tribonucleation.

In the tribonucleation explanation, there’s a thin film of viscous liquid between the two bone surfaces in the MCP joint. As the two bone surfaces are pulled apart during knuckle cracking, viscous adhesion or tension in the liquid film resists the pull but is then defeated. That defeat creates negative pressure that snaps the viscous liquid apart, creating a vapor chamber (aka a bubble) plus a cracking noise.

Boutin’s ultrasound imaging may back up the bubble creation theory. He thinks the bright flash seen in the images is from the liberation of gas from liquid, not from a popped bubble. The pressure changes created by the knuckle crack may spark more gas to come out of the liquid surrounding the joint. More work is obviously needed to settle the question and understand the exact timing of events.

One point that does seem to be snapping into clarity: cracking does not appear to be bad for your joints. In Boutin’s study, regular knuckle crackers didn’t have increased pain, swelling, or joint disability when compared with non-knuckle-crackers.