What is THAT?!

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Zac Efron left fans in frenzy when he uploaded a red hot topless snap on Instagram yesterday.

Zac Efron

Credit: Getty

But some shocked followers had to look twice thanks to an optical illusion making it look like the Hollywood hunk’s manhood was poking out of his shorts.

The movie heartthrob can be seen kicking back in a sauna, wearing nothing but a pair of white shorts, smouldering at the camera for the edgy pic.

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♨️ 🙏 @clearlightsaunas100

A post shared by Zac Efron (@zacefron) on

After spotting what appears to be a private part of Zac’s anatomy escaping from his trunks, fans went wild in the comment section.

‘I had to do a quick little double take and zoomed in on this picture!👀🤷♀️🤣😂,’ wrote one.

‘Um okay so do you see what I see?’ added another, while a third asked, ‘What is coming out of those shorts?’

MORE:Emily Ratajkowski lounges entirely naked in her living room for insanely sexy Insta pic

Clearing up the confusion, plenty more fans chipped in to explain the strange looking thing was simply a string from the former Disney star’s clothing.

‘Omg, that black string on your shorts could second for something else. Kinda in the craziest position. Lol,’ penned one.

‘It’s the string on his shorts. Lol I thought the same thing,’ a second remarked.

A third piped up to add, ‘If that’s how your mind works 🤷♀️ then crack on. But it’s clearly the loop string around his trousers.’

While many people may still associate Zac with playing teenage basketball pro turned performing arts lover Troy Bolton in the Disney classic High School Musical, he actually had his 32nd birthday earlier this month.

Sharing a dashing black and whit photograph of himself grinning at the camra while sporting a suave suit, the actor who recently played serial killer Ted Bundy thanked his supporters for their kind birthday messages, writing, ‘Thanks for all the bday wishes, love you all! ❤️