If you tuned into the Rio 2016 Olympics on Sunday and caught a glimpse of 19-time gold medal winning swimmer Michael Phelps, you may be wondering what those mysterious, circle-shaped bruises were on his back.

Turns out, those purple circles are a result of an ancient Chinese healing practice known as “cupping.” According to WebMD, cups are placed on the skin to create suction, which supporters “believe mobilizes blood flow to promote the healing of a broad range of medical ailments.” Cupping therapy can be used to treat blood disorders, high blood pressure, anxiety and depression, among other uses.

Thanks @arschmitty for my cupping today!!! #mpswim #mp @chasekalisz

A photo posted by Michael Phelps (@m_phelps00) on

According to The New York Times, “practitioners of the healing technique — or sometimes the athletes themselves — place specialized cups on the skin. Then they use either heat or an air pump to create suction between the cup and the skin, pulling the skin slightly up and away from the underlying muscles.” The capillaries underneath the suction cups then rupture, causing the circular bruises to appear.

Many of Team USA’s Olympians have used the practice, especially those in swimming and gymnastics. According to SB Nation, other Olympians that have tried cupping therapy include Alex Naddour (who found his $15 solution on Amazon), Pavel Sankovich and twelve-time Olympic medalist Natalie Coughlin. Naddour said in an article on USA Today that cupping has “been the secret that I have had through this year that keeps me healthy. It’s been better than any money I’ve spent on anything else.”

Of course, the most recent practitioner is Phelps, the most decorated Olympian of all time with 19 gold medals and 23 overall. The practice was even featured in an Under Armour video earlier this year.

Despite its popularity, The New York Times article goes on to say that there’s not much science to actually back up the claim that cupping has any real physiological benefits, or whether the practice simply has a placebo effect on the athletes. Studies have found cupping therapy to reduce pain, however, more research is needed to determine its benefits.

Twitter Reacts to Cupping at the Rio 2016 Olympics

Of course, Twitter had plenty to say about the circular bruises present on many of the Olympians. Many wondered what the bruises were, while others left plenty of room for jokes.

What are your thoughts on cupping therapy?