Michael Phelps' blotchy, red back on the pool deck in Rio de Janeiro outshone Team USA's gold medal performance in the 4x100m freestyle relay on Monday.
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The cause for the giant love bites seen on the most decorated Olympian?
Cupping.
A treatment made famous by actors Gwyneth Paltrow and Jennifer Aniston about 10 years ago.
The champion swimmer is a fan of the ancient Chinese healing technique, in which small jars are placed on the body to increase energy flow and release tension.
Dr Romy Lauche, a chancellor's post-doctoral research fellow at the University of Technology's Australian Research Centre in complementary and integrative medicine, said there were about five different methods, including blood cupping where the skin in punctured to draw out toxins.
Others include having an acupuncturist soak a cotton ball in alcohol, lighting it on fire inside a glass cup before removing the flame and placing the cylinder on the skin.
"Dry cupping" is, according to Dr Lauche, the most popular.
"Cupping is all based on unsubstantiated claims. No one knows what it actually does," she said.
Over the years Dr Lauche has been involved in several studies that investigated the benefits of cupping.
"One in particular looked at neck tension. We tested four cupping techniques and saw that subjects only felt a difference after five or more treatments."
Patients who took part in her most recent trial in Germany reportedly saw a decrease in tension and pain.
Phelps' adoption of the method is featured in a new advertising campaign for activewear label Under Armour. In the commercial he is seen swimming laps, enduring an ice bath, laying in a hyperbaric chamber and screaming during cupping therapy.
He has also documented other swimmers performing cupping on him in the change rooms at other swim meets on Instagram, like Olympian Allison Schmitt.
Phelps is not the only American athlete in Rio branded by what look appear to be giant, circular bruises.
Gymnast Alex Naddour's shoulders were covered in cupping marks when he took to the pommel horse over the weekend. Ahead of the games, Naddour bought his own set of cups online to maintain the practice, which reportedly keeps him "healthy" according to the New York Post.
Captain of the US gymnastic team Chris Brooks told USA Today many squad members had been relying on cupping inside the athletes village as trainers and doctors were not needed to conduct the procedure.
