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It's called "penis captivus." It can last for hours. And no, it isn't awesome.

Over the weekend, we told you about an Italian couple who reportedly started having sex in the Mediterranean Sea and finished in the ER, because their genitals wouldn't disunite. It sounds outlandish -- hell, we can barely figure out the physics of intercourse in the bathtub -- but there's an actual medical explanation for it. And the basic concept is pretty much this:

It's caused by a vaginal spasm, and apparently in the Italian couple's case, the water added a significant increase in suction. We asked Dr. Yvonne Fulbright -- formerly Astroglide's go-to sexologist, who now has her own line of products -- to pull us in and grip us with her knowledge...

Penis captivus started as an urban legend a long, long time ago

"Just hearing the term 'penis captivus' can have one shuddering," says Dr. Fulbright. "Penis captivus is an idea that started in medieval times. People feared that the penis could get 'sucked into' the vagina during the missionary position, possibly forever."

Legit medical studies have found evidence to support the theory

"While largely hearsay, and chalked up mostly as mythical, an article published in a 1979 issue of the British Medical Journal claims that penis captivus has occurred very rarely throughout history. Apparently, a handful of couples have gotten locked inseparably during sexual intercourse. This is actually pretty common [between] dogs."

Related: 22 Facts To Make You Feel Better (Or Worse) About Your Sex Life

Unless you're a dog, it's unlikely you'll ever experience this -- you have a better chance of winning the lottery -- but just in case, here's some advice...

"Should you ever find this actually happening, then both you and your lover need to relax and take the focus off of intercourse and anything sexy. This allows blood to leave the penis, so that he can withdraw more easily. And her pelvic floor muscles will also relax, as not to clench the shaft."