Ever wondered what a 20-year-old Macca's burger looks and smells like?

David Whipple has the answer for you.

After buying a hamburger from his local McDonald's in Logan, Utah, back in July 1999 for a measly 79 cents - he has kept it for more than 20 years for research reasons.

"I had a hamburger that I was using to show people about how things deteriorate," he told 2News.

He carried it around for about a month before stuffing it in a coat pocket and forgetting all about it.

"The coat got thrown in the back of my van, and I guess it just got hung up on our closet," he explained. "Subsequently, we moved from Logan to St. George, Utah, and it stayed there for a couple years, and I think my wife was giving the coat away or something and found it."

That was in 2013 when the burger was about 14 years old. At that time, Mr Whipple was interviewed about his ancient burger and his story went viral, having more than 10 million YouTube views.

Now, more than six years on, he has done it again with his zombie burger going viral all over again.

He kept it in a tin since rediscovering it all those years ago and hadn't opened it since - until he was interviewed again by the network.

"All right, I have no idea, maybe it's turned to dust," he told 2News.

He held up the paper it was wrapped in, saying "it has disintegrated more than the burger probably".

"There it is, my gosh. It hasn't changed. The pickle is the only thing that has disintegrated," Mr Whipple said.

According to Mr Whipple, the burger only had a faint smell of cardboard, describing it as "crazy" and a "real conundrum".

The Utah man is hoping someone will offer him a lot of money for it "so I can retire". But until then, he has packed it back in its tin.

Many took to Facebook to comment on the ageless burger, with the majority shocked with its youthful look.

"He found the fountain of youth, now we just have to bottle it," one woman wrote.

"Well, think how well you can be preserved if you eat this stuff," another said.

"Plot Twist: the burger is growing cancer-defeating bacteria!" a third commented.

A disgusted user wrote: "Why I don't eat fast food. So gross!!!!"

To put into perspective just how long Mr Whipple has had the Macca's burger, it was purchased around the time former US president Bill Clinton survived being impeached and the first Star Wars prequel was the number one movie.