Rio de Janeiro, BR…Maybe it came from the team’s group music video, “Carpool Karaoke,” which has drawn 4.6 million YouTube views in the first 12 days since going online. Maybe it came from the cowbell ringing in the warm-up pool to salute each U.S. swimmer as he or she went to the ready room before a race.

Maybe it came from assistant coach Greg Meehan’s history-lesson-cum-motivational-ploy of having each of the women swimmers plant an American flag on grass near their building in the Olympic village, claiming the land for their own the way the 1862 Homestead Act had encouraged settlers to move West.

Maybe it came from the “ice-breaker games” Meehan, the Stanford women’s head coach, had the team play at their pre-Olympic training camps in the U.S. Those games were designed to last five minutes but sometimes turned into 45 minutes of belly-laugh bonding.

Maybe it came from the positive vibes created as swimmer after swimmer had stunning performances in practice at those camps in San Antonio and Atlanta.

Or maybe it was all those reasons, both intangible and real, that explain how 47 athletes in an individual sport created a team that utterly – and a bit surprisingly – dominated the eight days of Olympic swimming that ended Saturday.

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“It’s going to go down as one of the greatest Olympic performances ever, and we weren’t forecast for that,” Bob Bowman, U.S. men’s head coach and Michael Phelps’ personal coach, told NBC.

How great?

*With 33 medals, the U.S. had 23 more than runner-up Australia and three more than the COMBINED total of countries two through five in the standings.

*Although the U.S. also won 33 medals in 2000, its share of available golds (50 percent) and total medals (56.9) percent was better (gold) or as good (total) than any in the eight straight non-boycotted Olympics since each country was cut from maximum three entrants to maximum two per individual event after 1976. The only better U.S. percentage since 1976 came with the Soviet Bloc countries, notably East Germany, absent from the 1984 Los Angeles Games.

In Rio, the U.S. saw the reconfirmation of one superstar, Michael Phelps (five golds, one silver), and the creation of another, Katie Ledecky (four golds, one silver, two individual world records and a match of compatriot and Mexico City 1968 heroine Debbie Meyer as the only women to win the 200, 400 and 800 freestyles at the same Olympics.)

It saw breakthroughs by sprint freestyler Simone Manuel (two golds, two silvers, first African-American woman to win individual Olympic gold) and backstroker Ryan Murphy (gold in both backstrokes, 100 world record on the backstroke leg of the medley relay after missing the team in 2012.)

Fittingly, what would be the 1,000th Olympic gold medal for Team USA in summer competition came from swimmers: the women’s medley relay anchored by Manuel in the meet’s penultimate event.

All that came after performances barely a month ago at the U.S. Olympic Team Trials for Swimming that were decidedly underwhelming.

“We had less than ideal conditions at trials,” U.S. women’s coach David Marsh explained in a voice message Saturday.

Marsh cited crowded pools and environmental challenges (too much chlorine in the warm-up pools), plus pressure from trying to make the team in front of bigger, more boisterous Omaha crowds (every session sold out) than those who failed to fill all the seats in Rio.

“The word I had from the swimmers after trials was relief – joy, certainly, but a lot more relief,” Marsh said. “One day here, Katie Meili looked at me from the warmup pool and said, This is fun,’ while Olympic Trials was nerve-racking.”

Meili would win an unexpected bronze medal in the 100 breaststroke with a Rio time nearly one-half second faster than her time in Omaha. Others were similar overachievers.

Even as many athletes failed to swim their best at the trials, Marsh knew immediately the United States was sending people to Rio who would be the best catalysts for creating a team chemistry that fused 31 Olympic rookies with 16 veterans – one of whom, the 19-year-old Ledecky, was once again the youngest on the team.

“As people like Elizabeth Beisel (three Olympics) and David Plummer (rookie, but a 30-year-old father of two) made it, people who are giant adds to a team culture, we know they are always going to be as much about the team as themselves,” Marsh said.

Beisel, a silver medalist in 2012, said at the first team meeting, “I swim Day 1, and after that I’m going to give you guys everything I have.”

After her sixth in the 400-meter individual medley, the outgoing Beisel was always in the stands or behind the scenes leading cheers. Despite having finished competing and being what Marsh called basically retired from the sport, Beisel also kept eventual 4×200 freestyle gold medalist Allison Schmitt company in the warm-up pool several days.

On the surface, the concept of team in a sport where 26 of 32 Olympic events are for individuals does not seem obvious – or that important. Yet U.S. swimmer after U.S. swimmer brought up the idea in Rio.

Ledecky: “Team USA ha been doing great, and we feed off each other’s energy.”

Manuel: “We’re a family, and it’s a great support system.”

Josh Prenot (breaststroke silver): “It’s a huge thing. I’ve never seen this on the national team before.”

There is no doubt the feeling of collective purpose was a paramount reason for U.S. swimmers’ consistent and near totally inclusive success in Rio, marked by statistics like this:

*The U.S. had no finalist in just one of the 26 individual events, the women’s 200 breaststroke.

*Both U.S. entrants made the final in 12 of the 13 men’s individual events and in seven of the 13 women’s.

*The U.S. won medals in 29 of the 32 events, the highest success percentage since the Olympic swimming program reached 32 events in 1988. It was 27 of 32 in London.

*The U.S. also had five near misses in fourth place.

Such results began to seem much more likely during the post-trials training camps than they had at the trials.

“Everyone made good adjustments during the camps,” Marsh said. “Several people had phenomenal training sets, and I think that gave everybody confidence.”

Phelps was among those whose practice performances in front of teammates at those camps were a motivating harbinger of what he – and other U.S. swimmers – would do in Rio.

Yet his medals, increasing his mind-boggling Olympic record totals to 23 gold and 28 overall, were just one of his contributions to the U.S. numbers.

Phelps’ becoming just the second swimmer to carry the U.S. flag at the Opening Ceremony – after Gary Hall in 1976 – turned into a celebration for his swimming teammates. They already had elected him one of three men’s captains.

“Michael was one of the keys to why we are what we are here,” Marsh said, referring to the medal totals. “(It) wasn’t only his being elected captain for the first time ever on a national team but also (being) a darn good captain.

“A lot of his most effective moments were in the warm-down pool. Somebody jumps in the lane, and he just picks up casual conversation with them. To have Michael Phelps take his time to personally know how you did your race has a lot of power, especially for the younger swimmers. And he certainly did that a lot.”

Phelps is at the wheel a few times in the carpool karaoke video. Its playlist includes the them from Pokémon, a game that was part of these millennial swimmers’ childhoods before returning last month and becoming a national obsession.

The U.S. swim team did more than just sing along. It called the tune.

The music might as well have been the triumphal chorus from Verdi’s “Aida,” with its talk of laurel wreaths on the victors’ heads, like the prizes in the ancient Olympics. A chorus is by definition a group of individuals acting in concert, like U.S. swimmers becoming a team.

And, after all, the opening lines of the Pokémon song are “I wanna be the very best / Like no one ever was.”

Maybe it came from that.

Philip Hersh, who is covering his 18th Olympic Games and was the Chicago Tribune’s Olympic specialist for 30 years, is a contributor to TeamUSA.org.