Is Julianna Pena’s destruction of Amanda Nunes really the biggest upset in MMA history? (VIDEO)

If Julianna Pena's seismic upset win against Amanda Nunes taught us anything it is that in MMA, the bigger they come, the harder they fall - and this has been a lesson learned the hard way by several fighters down the years.

There's a well-worn saying in mixed martial arts: if you don't have a loss on your record, then you're night fighting the right level of competition.

While supporters of Khabib Nurmagomedov might have their issues with statements such as this, it has remained an almost inescapable truth for practically every other fighter to have stepped into a cage over the years.

Nunes, the UFC's first-ever female double champ, experienced this for herself against Pena last weekend at UFC 269 as she shockingly surrendered her bantamweight crown to her underdog opponent in Las Vegas.

READ MORE: ‘I told you motherf**kers!’ Pena submits Nunes for ‘biggest upset in UFC history’ (VIDEO)

The upset is being considered by many as the biggest seen in the Octagon in some time, and possibly ever – but it is far from the only seismic shock to hit MMA, with several others rocking the sport throughout its short history.

MATT SERRA VS. GEORGES ST-PIERRE – UFC 69, APRIL 2007

It wasn't supposed to go down like this.

St-Pierre, six months or so removed from what seemed to be the realization of his destiny when he dethroned dominant UFC welterweight champion Matt Hughes, was handed what many saw as a 'gimme' fight against Serra, a journeyman fighter and former lightweight who had earned the opportunity by outlasting several other veterans on a season of the UFC's reality show 'The Ultimate Fighter'.

Instead, we got what is still hailed among the most shocking finishes to a fight in UFC history.

Serra, a prohibitive underdog against the outstanding St-Pierre, refused to bow to the oddsmakers and immediately launched into a furious assault against the 170lbs champion.

GSP was floored on more than one occasion as Serra swarmed, with St-Pierre eventually tapping out due to strikes as the aggressive New Yorker unloaded an endless barrage of punishment on the grounded French-Canadian.

The result of the fight, and the manner in which it was achieved, is said to have compelled St-Pierre to forevermore adopt a more conservative strategy in his fights, but not before gained his revenge – and the world title – from Serra in their rematch a year later.

He would never lose another fight in his professional career before retiring a decade later.

HOLLY HOLM VS. RONDA ROUSEY – UFC 193, NOVEMBER 2015

MMA neophytes might not grasp exactly how big a star Ronda Rousey was throughout her four-year unbeaten run through Strikeforce and the UFC.

The former Olympic judoka finished each of her career wins, all but one of those coming inside the first round – and nine of them coming by armbar. 

Her rapid ascent to the summit of the sport brought with it a selection of hot takes: her boxing is as good as anyone in the sport (it wasn't), she could quite likely beat the male 135lbs champion (she almost certainly couldn't), and she would never feel the sting of defeat as long as she remained an active fighter.

That last statement met a grisly end in Melbourne, Australia, in late 2015 when she faced fellow undefeated fighter and professional boxer Holly Holm – someone who the opening exchanges showed was vastly more comfortable on the feet.

The end, when it came as the result of a shuddering left high kick to the dome, was almost merciful, as if putting Rousey out of her misery and forever dispelling the narrative that female fighting had never seen as dominant a champion.

Rousey would also lose her next fight to Amanda Nunes before emerging in the pantomime world of WWE sometime later.

FABRICIO WERDUM VS. FEDOR EMELIANENKO – STRIKEFORCE: FEDOR VS. WERDUM, JUNE 2010

The evening of June 6, 2010, was thought by most to be just another chapter in the great Fedor Emelianenko's legacy.

The dominant Russian heavyweight had, for almost a decade, defeated all comers and was unbeaten in 33 fights, save for a technical defeat due to a cut in a one-night tournament in late 2000. 

Submission specialist Werdum was a test, many thought, but one that the great Fedor was certainly favored to pass. After all, the Brazilian submission ace had washed out of the UFC a couple of years prior after a mostly underwhelming 18 months with the organization.

But just 69 seconds into the contest, Fedor tapped out after being baited by Werdum into a tight triangle choke in what was the first true defeat of The Last Emperor's glittering career... and it opened the door to two more immediate losses against Antonio Silva and Dan Henderson.

Emelianenko, now 45, has lost just two more times since that trio of defeats while winning nine times, including an October knockout of American Tim Johnson in Moscow. 

NATE DIAZ VS. CONOR MCGREGOR - UFC 196, MARCH 2016

Nate Diaz declared that he wasn't "surprised" after he handed Conor McGregor his shocking first UFC defeat just months after the Irishman had vanquished featherweight boogeyman Jose Aldo to claim his first of what would eventually become two world titles – but the rest of us sure were.

McGregor moved up to the 170lbs division to take on Diaz on short notice after lightweight champion Rafael Dos Anjos withdrew from their scheduled bout due to injury, and was expected to breeze past the tough but beatable veteran.

Instead, McGregor tired himself out by emptying his glitzy box of striking tricks and found himself eventually enveloped in Diaz's quicksand, a second-round rear-naked choke. 

McGregor would go on to claim redemption the following August when he outpointed Diaz in what was a thrilling contest – but it was the Stockton fighter who first exposed the chinks in the Irishman's armor all those years ago.

RANDY COUTURE VS. TIM SYLVIA – UFC 68, MARCH 2007

"Not bad for an old man" - Randy Couture

The UFC got its very own iteration of George Foreman vs. Michael Moorer in March 2007 when the 43-year-old former light heavyweight champion Couture emerged from retirement to put a five-round beating on then heavyweight kingpin Tim Sylvia, a fighter who was 23-2 at the time and stood some seven inches taller than Couture.

Couture's comeback was bemoaned by many as sending a beloved former fighter to the slaughter. He had lost two of his prior three fights by crushing knockout to Chuck Liddell, so how was he going to be equipped in handling the power of a fighter who was both younger and bigger than him?

Quite well, as it turned out. Couture floored Sylvia with his opening salvo, an overhand right, and didn't step off the gas for the remaining 24-and-a-half minutes of the fight to claim the UFC's world heavyweight title.

Couture would go on to compete seven more times in the UFC before finally retiring at the age of 48 in 2011, but not before he well and truly made us reconsider the shelf life of a top fighter.

Not bad indeed.