(CNN) Street art has come a long way from its humble and rebellious beginnings. It has become popular the world over (just check the number of Instagram tags) and is exhibited in galleries -- with the likes of Banksy and JR becoming celebrated artists in their own right.
And while it's still associated with illegal graffiti, street art has increasingly become the subject of commissions from public bodies.
Nomad Clan are an internationally acclaimed street art duo from the UK, who have painted giant murals all over the world, commissioned by cities, galleries and non-profit organizations.
The "Athena Rising" mural in Leeds is said to be the tallest in the UK. It's the work of British street art duo Nomad Clan.
Their work often tackles social and environmental issues. "Pipe Dreams," in Arvika, Sweden, depicts an old angler with a dream of endless fish. "It's easy to live under the illusion that our lifestyles aren't having a negative impact on the environment," says Nomad Clan.
Joy Gilleard, known as Cbloxx, is one half of Nomad Clan. "I think one of the things we do is mash up the storytelling element with some of the heritage stuff, maybe social issues that we're touching on. But we're presenting it in a way that everyone can enjoy on a visual level," she says.
Hayley Garner, known as Aylo, is the other member. "We usually start with massive research of where we're going, what the place looks like, if there's any folklore surrounding the area," she says.
Their work "Mokosz," in Lodz, Poland, depicts a Slavic goddess from traditional Polish folklore.
"True North," in Leeds, makes reference to the historical importance of mills and industry in northern England. The figure on the left is "Jimmy Boy," Aylo's grandfather, who worked for years in a mill in the town of Rochdale.
"Sentient Beings" shows an octopus and a human. Both are intelligent creatures, explains Nomad Clan, but "one lives in harmony with its environment, the other disrupts the balance."
"King of Nowt," ("nowt" is slang for "nothing") in Manchester, is meant to raise awareness of high rates of suicide among young men in the UK. It illustrates someone conforming to a stereotype of a strong male.
"Still Waters," in Belfast, is described by Nomad Clan as "a nod to Belfast's ship building heritage, its linen and its rope industry. The wolf is on Belfast's shield. Wolves were once an integral part of the Irish countryside and culture."
"Final Fleet," in Blackpool, UK, is a tribute to the coastal town's diminished fishing industry.
Nomad Clan at work in their studio. "The most intense part [of the work] is getting the sketch down and getting the sketch right, because that's the framework that you build from," says Arlo.
In 2017 they created what's been called the UK's tallest mural, in the city of Leeds. "Athena Rising" depicts a swooping owl -- an animal associated with wisdom and the ancient Greek goddess Athena.
It took them seven months to plan and a month to execute, painting for eight hours a day from a movable platform suspended from the building, and using just over 300 cans of spray paint.
The mural is more than 150 feet (46 meters) tall and 36 feet (11 meters) wide. That's taller than the Statue of Liberty, from head to toe.
But size isn't everything: from a poignant piece in London's St. Paul's cathedral, which highlighted the refugee crisis, to organizing workshops in a Calais refugee camp, Nomad Clan's work often tackles social, environmental and humanitarian issues.
A work created by Nomad Clan and migrants at the "Jungle" camp, in Calais.
"I think street art has always been about putting a message out there, questioning things and getting people thinking and talking," says Joy Gilleard, one half of Nomad Clan, who goes by the name Cbloxx.
"Just because we paint giant public art murals doesn't mean that we want to remove those elements from our work."
Find out more about Nomad Clan in the video above.