They are the family behind the biggest department store, but somehow, the Walton clan have largely flown under the radar for decades.

That's despite being the world's wealthiest family, with a US$191 billion (NZ$297 billion) fortune.

And according to Bloomberg, it's a fortune that's growing — fast.

"The numbers are mind-boggling: $70,000 (NZ$108,000) per minute, $4 million (NZ$6.2 million) per hour, $100 million (NZ$155 million) per day," the publication reports.

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"That's how quickly the fortune of the Waltons, the clan behind Walmart Inc, has been growing since last year's Bloomberg ranking of the world's richest families.

"At that rate, their wealth would've expanded about $23,000 since you began reading this. A new Walmart associate in the US would've made about 6 cents in that time, on the way to an $11 hourly minimum."

It also revealed the family's "jarring" and "near-unprecedented" wealth had soared by US$39 billion since June 2018.

But who are the mysterious Walton family, and how did they amass such a "jarring" stack of cash?

Sam Walton was famously frugal, allegedly only flying first class once, living in a modest home and driving a beat-up red pickup truck.

But Rob, his eldest child, seems to be more happy to splash his cash.

According to Business Insider, he collects valuable vintage cars as a hobby — and in 2013, he managed to total his Daytona Coupe.

Walmart chairman Rob Walton in 2015. Photo / AP
Walmart chairman Rob Walton in 2015. Photo / AP

It was one of just five in the world and was worth a whopping US$15 million.

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When John Walton died in a plane crash in the mid-noughties, he left around 17 per cent of his fortune to wife Christy, while the rest was divided between various charities and his son Lukas.

Jim Walton, 71, has an estimated net worth of $US49.6 billion and is married to Lynne McNabb Walton.

The couple have four children — Alice A Proietti, Thomas L Walton, James M Walton and Steuart Walton, 38, who is a is a Walmart director.

The latter married Baywatch star Kelly Rohrbach earlier this year.

According to Page Six, "Rumours swirled in March that they were engaged, after author Cat Marnell tweeted that she overheard Walton 'call fiancee Kelly Rohrbach (and) then bitch out … their wedding planner'. She claims to have eavesdropped on Walton saying that their wedding budget was "$1 to $1.5 million".

He and brother Thomas are keen mountain bikers and they co-founded an investment company, RZC Investments.

Alice Walton has a net worth of around $US48.7 billion and has been divorced twice with no children.

Alice Walton is a keen supporter of the arts. Photo / AP
Alice Walton is a keen supporter of the arts. Photo / AP

She's a lover of the arts and has amassed a huge personal collection worth US$500 million, according to Business Insider.

In 2014, she made history after paying US$44.4 million on a Georgia O'Keefe painting — the most ever spent on an artwork by a female artist.

She's also bought and sold a string of luxury, multimillion-dollar properties over the years, including two sprawling Texan ranches and a condo on Park Avenue in New York.

She also donated millions of her Walmart shares to the clan's not-for-profit organisation, the Walton Family Foundation.

According to Bloomberg, the family-owned Walton Enterprises owns half of Walmart, and that 50 per cent stake paid out US$3 billion in dividends last year alone.

Somehow, the clan have managed to remain out of the spotlight for the most part, with family members steering clear of social media and avoiding being papped in public despite having a Hollywood star in the ranks.

The Walton family tree

Sam Walton married Helen Robson in 1943

They had four children - Samuel (Rob) Robson, John Thomas, James (Jim) Carr and Alice Louise

He founded Walmart in 1962

Sam Walton died in 1992, leaving ownership of the company to his family

Rob took over as chairman, and John served as a director before his death in 2005

He left behind wife Christy and one son, Lukas

Jim and Alice are not directly involved in the company, although Jim is chairman of the board of the Walton's Arvest Bank

His son Steuart is on the Walmart board, and has three other children: Thomas, James and Alice

Alice is divorced with no children, while Rob is believed to have at least four children from three marriages.