Some appear as if out of thin air. Others can be scheduled, as in Paris where on the opening night of competition on Sunday morning (NZT) three world record holders could get the Summer Games off to an explosive start in a women's 400m freestyle showdown already being hyped as a "Race of the Century" candidate.
The United States' Katie Ledecky, already rated one of the greatest swimmers of all-time, Australian torpedo and current world record holder Ariarne Titmus, and Canadian teen sensation Summer McIntosh are set to light up the Paris night in a race for gold that may produce a swim for the ages.
"I'm a student of the sport," Ledecky said. "I keep track of everything that's going on around the world.
"Ariarne is incredible, what she did last week (Australian Olympic trials) and what Summer's done it is going to be great.
Fairweather threat
Another swimmer, who won't have the same spotlight on her but will want to be in the scrap for medals is rising Dunedin freestyler Erika Fairweather.
With the big names missing, 20-year-old Fairweather raced to gold at the world championships in February, backing it up with silver in the 200m.
While she openly conceded victory in Doha came with an asterisk, Fairweather did enough with her national record time of 3 minutes 59.44 seconds to put her rivals on alert.
Competing at her second Olympics, she said she felt relaxed and confident after arriving in Paris from her training base in Monaco.
"The 400 free is always one that's hyped up for the women. We've got some of the most incredible athletes in that one race, so I'm ready to give it a good crack," she said.
"The main part is concentrating on your own race. You have your own 50m lane and you can't worry about what everyone else is doing. The only thing you can control is how you swim on the day.
"I have had an amazing training block over the last 12 months. Even post-qualifying to now, training's been really good so I'm really excited for racing."
Personalities
Personalities make compelling rivalries and the three favourted women, all in different stages of their careers, come to Paris giving off different vibes.
Ledecky is the elder stateswoman, a seven-time Olympic gold medallist who chooses her words carefully not wanting to provide added motivation in a race that could be decided in the blink of an eye.
McIntosh, who made her Olympic debut in Tokyo at just 14-years-old, has matured into a poised competitor but her confidence in the pool is contrasted by an uneasiness in the spotlight.
"She (Ledecky) was constantly on my mind," Titmus said. "But honestly, not really (any longer) because, I know that my best foot forward is the best, so I'm just going to do that, put my best foot forward.
"The 400 is Day One. Everyone's going into it fresh. No one knows what form anyone's in.
"For 400 I feel like out of all my races I know how to race it the best."
What the trio have in common is they all swim fast. Very fast.
Ongoing rivalry
At different times Ledecky, Titmus and McIntosh have owned the 400m free world record, and combined they have posted the top 28 times ever in the event.
Ledecky claimed the 400m free gold at the 2016 Rio Olympics with Titmus knocking the American from top spot on the podium into second place in Tokyo.
"It's pretty incredible that three of the previous or current world record holders are swimming in the event together," said McIntosh, looking ahead to the 400 free.
"So yeah, I'm really excited going in and since it is Day One, it's a great way to start off the swimming for sure.
"Right now, I'm just focused on myself like I always have been. "I've got three other individual events. The 200 IM is going to be a crazy race as well so just trying to take it one race at a time."
- Additional reporting RNZ