Here's why Michael Phelps was staring daggers at Chad le Clos

It was the face that launched a thousand memes. Minutes before the 200m butterfly semifinals on Monday, a hooded Michael Phelps put on his best Miz girl impression and stared daggers through South African swimmer Chad le Clos' back. Minutes later, le Clos returned the favor at the starting blocks and at the pool, glancing back at Phelps whenever he had the opportunity, his eyes stuck somewhere between between puppy dog and great white shark.

It was an instantly memorable moment, meant to go viral from the minute it was caught by NBC's cameras. Phelps, the greatest swimmer the world has ever seen, stared down his rival like a child sizing up a plate of broccoli. Le Clos, the man who derailed his bid for gold in the event four years ago, remained oblivious as he shadowboxed his way through his warmup, then focused singularly on the American once they approached the pool. Sure, Hungarian Tamas Kenderesi may have wound up winning the heat, but for all intents and purposes, there were only two men in the water on Monday night.

So, how did we get here?

The le Clos-Phelps rivalry began in earnest at the 2012 Olympic Games. The South African, then 20 years old, shocked the world by upsetting the venerable veteran in London in the 200m butterfly, touching the wall 0.05 seconds ahead of Phelps to claim his first gold medal. It was a stunning win, and Phelps' first international loss in the event in a decade. Even so, the pair remained cordial in the aftermath. Phelps walked off into retirement and le Clos stepped into the spotlight as his likely successor in the event. The two were even friendly enough to plan a shark cage diving trip together in 2013.

When Phelps announced his return to the pool in 2014, the 200m butterfly wasn't even on his agenda. However, he took a look at the times the world's elite swimmers were posting in the event and decided the opportunity was there to win gold once more. It was a decision that did not sit well with le Clos.

Exactly one year before Monday's stare down, le Clos fired the shot that created the Olympics' most memorable stank face. After winning gold in the 100m fly at the 2015 World Championships -- an event Phelps was barred from competing in after a DUI arrest in September 2014 -- the South African dismissed his natatorial rival. "Michael Phelps has been talking about how slow the butterfly events have been recently," he said. "I just did a time he hasn't done in four years. So, he can keep quiet now."

The American's reaction? "Chad liked me, and then he didn't like me," Phelps said in a New York Times story. "He said I was his hero, and then he was calling me out." And while he may have been laughing when he talked to the press earlier this year, there was no smile on his face in Rio de Janeiro Monday night.

The best part? Phelps-le Clos may not even be Olympic swimming's best beef in 2016. Lilly King called out Russia's Yulia Efimova for her two positive tests for banned substances on Sunday, then beat her to earn a gold medal in the 100m breaststroke on Monday. Australian Mack Horton beat favored swimmer Sun Yang in the 400m freestyle on Saturday, then enraged seemingly all of China by bringing up Yang's failed PED test in 2014 and suggesting he didn't "have time for drug cheats."

So, while the heat between Phelps and le Clos may be very real, it isn't especially unique at the Olympic Aquatics Stadium in 2016. The pair meet again on Friday night in the 100 fly after Phelps bested everyone, including le Clos, in the 200 earlier in the week.

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The history of the Phelps and le Clos swimming feud