Millions — perhaps tens of millions — of shipping containers are sitting empty at ports all over the world. And they've been a treasure trove for architects Ada Tolla and Giuseppe Lignano.

"We found so many — it felt like something so ripe to pick, basically," said Lignano. He and Tolla were in San Francisco recently for the opening of an art exhibition at Hosfelt Gallery focused on their use of shipping containers as building material and art project.

The Italian "starchitects" got into the shipping container building game in the 1990s, roughly a decade after these types of buildings first started appearing. (Shipping containers were invented in the mid-1950s, but the first reported instance of shipping containers being converted into housing was 1987.)

Lignano and Tolla's New York-based firm LOT-EK's projects include an experimental art school in New Orleans for people of color and an affordable housing complex in inner-city Johannesburg, complete with swimming pool.

People like shipping container buildings not only because they look interesting but also because they seem to solve a problem — finding a use for the millions of empty steel shipping containers scattered across the planet. They're used in projects like Photoville in New York City, which transforms the containers into mini art galleries, and Monarch Village, a development for formerly unhoused people in Lawrence, Kansas.

"Shipping containers are great for building with because they are modular, movable and durable," said California architect Douglas Burnham. His firm, Envelope, created Proxy, a development in San Francisco that includes several businesses housed in shipping containers, from a clothing store to a beer garden.

Containers are also an attractive alternative to traditional construction materials such as cement — cement manufacturing produces the world's third-highest level of planet-warming pollution — and wood, which requires cutting down trees and growing them again.

Italian architect Tolla said she and Lignano favor containers that are 10 to 15 years old, both for sustainability reasons and because they like the containers' hip, dilapidated look.

"Beauty can be found in things that might look ugly," Tolla said.

Most people don't want old containers

But here's the thing: The vast majority of people in the market for an office, public facility or home made out of shipping containers don't buy them heavily used, because doing so doesn't make financial sense.

"When you're building a $100,000, $200,000 structure, that $1,000 to $2,000 difference between a new container and a used container is not really significant anymore," said Alex Rozkin, the CEO of Conexwest, a nationwide shipping container supplier. "And most customers will just opt for the new one."

Rozkin said most customers buy old containers only to build basic structures like storage units. And new — or nearly new, "one-trip" containers — come with additional benefits.

"They don't have the dents," Rozkin said. "They don't have the rust."

Also, some municipalities, like Los Angeles, won't allow the use of containers that are damaged, that have been previously repaired or that are more than two years old.

"If you're using a one-time-use container ... then that container would be put to better use transporting goods across seas and oceans, which is the purpose it's meant to serve," said architect and construction technology expert Belinda Carr in an episode of her YouTube video series.

"The idea that you are saving the environment when you use shipping containers and that it's a highly sustainable practice — I understand if you're using something meant for the landfill. But if you are using a brand-new shipping container, what's the point?"

Carr said another significant challenge is temperature regulation. Those steel boxes get very cold inside — and very, very hot.

Brooklyn, N.Y., restaurateur Joe Carroll commissioned and lived in an eye-catching shipping container home designed by LOT-EK's Tolla and Lignano for five years. The home is prominently featured in a new documentary about the architects' work, We Start With the Things We Find.

Carroll told NPR that he appreciated many things about LOT-EK's approach.

"It's about designing structures that are unique looking, not just a stack of cubes," said Carroll.

But Carroll also said his energy bills were sky high.

"There was no thermal heat or solar," he said. "We didn't have any of that in the home."

All that heating and cooling takes not only money but environmental resources.

So — what should we do with them?

Critics say the most environmentally friendly use of all these unused steel shipping containers is to recycle them.

"The pitch of these containers is, 'Well, we're saving them.' But it doesn't make any sense," said San Francisco-based architect Mark Hogan of OpenScope Studio, who has publicly shared his concerns about shipping container housing. "You'd be much better off recycling the container into steel and then build out of steel studs — like the normal way you'd build a building."

This story was produced for air by Isabella Gomez Sarmiento and edited by Jennifer Vanasco for digital and air.