Immediately after the world’s last male northern white rhino died on March 19th, a team of vets got to work. Within 30 minutes, they had collected tissue from the ears, gums, spleen, windpipes, and testicles of the 45-year-old rhino, named Sudan. The precious genetic material was put in a solution and then frozen at the Ol Pejeta Conservancy in Kenya, where Sudan spent the last nine years of his life. Those cells could one day bring the northern white rhino back from the brink of extinction.

Dozens of scientists across the globe — from the US to Europe to Africa — are working together tirelessly to figure out ways to breed rhino embryos in the lab. The effort resembles in some ways the popular de-extinction projects that are attempting to resurrect the woolly mammoth or the passenger pigeon; all want to reverse extinction and in some cases, fix the damage humans have done.

“I think it is our responsibility.”

The odds of success for the rhino are much higher: Unlike species that have been extinct for decades (or thousands of years!), northern white rhino DNA and sperm are preserved safely in different labs around the world. If it works, the project could bring back herds of northern whites that used to roam the grasslands of east and central Africa, where they were poached for their horns.

“They are at the brink of extinction only due to human activity,” says Jan Stejskal, director of communication and international projects at the Dvůr Králové Zoo in the Czech Republic, where Sudan lived until 2009. “If we have the techniques or methods to assist them to survive, I think it is our responsibility to utilize them.”

De-extinction has been a sci-fi trope for decades — but now the science may have finally caught up to our imagination. Today, projects like the Woolly Mammoth Revival led by Harvard’s George Church are trying to use biotechnology to resurrect the extinct species and repopulate the tundras and forests of Siberia and North America. It works like this: bits of mammoth DNA are edited into the genetic code of its living cousin, the Asian elephant. A hybrid embryo would then be grown in an Asian elephant surrogate mother — or an artificial womb, Church says — to give birth to a new mammoth-elephant animal.

Despite claims that the hybrid embryo could be created as soon as next year, the project is far from resurrecting herds of mammoths. It has, however, kicked off a heated debate about whether de-extinction technology should even be used. Many argue that the money spent to bring back long-gone species should be devoted to preserve the ones that are still around. Others criticize the ethics of resurrecting species whose habitats might be gone and putting surrogate mothers at risk.

Breeding a herd of northern white rhinos is estimated to cost as much as $9 million, according to the Dvůr Králové Zoo, with much of the money coming from donations and zoo revenue. The San Diego Zoo, which is also involved in the project, says an estimate is impossible since the technology needed is still being developed. “Over the course of three years the total annual budget has exceeded $1 million,” Stacey Johnson, corporate director of conservation and research at San Diego Zoo Global, tells The Verge in an email. But the northern white rhino project is fundamentally different from other projects like the Woolly Mammoth Revival, and that makes the money worth it, says Joseph Bennett, an assistant professor at Carleton University, who’s criticized the costs of de-extinction and is not involved in the northern white rhino project.

For starters, unlike the woolly mammoth, the northern white rhino is not extinct — yet. Only two females remain: Najin and Fatu, who are both related to Sudan and live at the Ol Pejeta Conservancy under armed surveillance. While the habitat of the woolly mammoth is widely different from what it was thousands of years ago — fragmented by roads and cities, for instance — the habitat of the northern white rhino still exists.

“It makes a lot more sense to me to work on something like this than it does something that’s been extinct 10,000 years, that might not even survive in a warming Arctic,” Bennett says.

Northern white rhinos have been extinct in the wild since 2008, but only because they were poached for their horns. And that makes humans responsible for their survival. “If ... we’re at fault for extinction, we’re also on the hook for coming up with a way to try to undo that extinction,” says Douglas McCauley, an ecologist and conservation biologist at UC Santa Barbara, who’s also been critical of de-extinction projects. McCauley agrees with Bennett that trying to rescue the northern white rhino is “wholly appropriate.” Rhinos play an important role in the environment, dispersing seeds and keeping vegetation — and as a result, rodents and snakes — under control. Plus, rhinos are going extinct right now. It’s more like playing Noah than playing God, McCauley says.

“It’s a little bit more clean ethically,” he says.

There’s one more key difference between northern white rhinos and mammoths: While we only have bits of mammoth DNA, we have plenty of complete genetic material — as well as sperm — from several northern whites. It’s kept securely frozen in labs all over the world.

In 1986, Oliver Ryder, director of conservation genetics at the San Diego Zoo Institute for Conservation Research, flew to the Dvůr Králové Zoo in the Czech Republic to collect Sudan’s skin cells. Sudan had been living there since the 1970s, as other northern white rhinos in Africa were being exterminated by civil wars and poaching. At the time, Ryder was studying the genetics of northern whites to determine whether they were a distinct subspecies from the southern white rhino, which is slightly bigger and lives in southern Africa. (They are.) Sudan had never been anesthetized before, and Ryder recalls, “There was a tension.”

The procedure went smoothly, and the cells were eventually added to the Frozen Zoo, the menagerie of cells, eggs, sperms, and embryos of about 1,000 species and subspecies, hosted at the San Diego Zoo. At the time, Ryder didn’t know that those cells could one day be the key to rescuing the animals. “I wasn’t that visionary, honestly,” he says. He did have an inkling they could be important in ways he hadn’t yet thought about, though.

In 2015, Ryder, Stejskal, and others met in Vienna, Austria, and put together a plan to use the genetic material and sperm taken from northern whites to reverse extinction. The plan involves a few options, with techniques that have been proven to work in humans and other animals, like mice, but not in rhinos. “There might be obstacles that could prove too difficult,” Stejskal says. “In the case of the rhino, you don’t do it because you’re sure that you will succeed, but you do it because you feel it’s the right thing to do.”

“you do it because you feel it’s the right thing to do.”

One option calls for taking the frozen sperm and using it to fertilize rhino eggs in a petri dish. But there are a few problems: While the technique has been used in humans, livestock, and most recently in bison, it’s never been done with rhinos, says Richard Vigne, the CEO of Ol Pejeta Conservancy. The second problem is that there are no banked northern white rhino eggs. Female rhinos are massive, and their ovaries are almost 5 feet inside their bodies, Stejskal says. So to reach the ovaries, scientists need to use a special tool with an ultrasound probe and needle at the end to aspirate the egg cells. “You have to operate with this tool very carefully,” Stejskal says. If the needle punctures what it’s not supposed to, “it could have really serious consequences.”

Stejskal and his colleagues at the Leibniz Institute for Zoo and Wildlife Research in Berlin have been working on over a dozen southern whites to optimize a method to harvest the eggs. (None of the females can have babies, so any screw-ups that could make them sterile won’t matter.) And similar work is going on at the San Diego Zoo. But it hasn’t been tried on Najin and Fatu yet. The two remaining northern white females are too precious for any major screw-ups, but the technique might be close to being ready: Stejskal says he’s hoping to attempt the procedure on Najin and Fatu in three or four months.

Meanwhile, there’s option number two: making rhino eggs and sperm in the lab. Theoretically, this can be done by turning frozen rhino cells into cells that can transform into any cell of the body, called pluripotent stem cells. Those stem cells can then be turned into sperm and eggs. Lab-made sperm is important because the frozen sperm collected directly from the animals might not be high quality. When Sudan’s sperm was collected in 2014, for example, he was already old, Stejskal says. But the lab-made eggs are the most important part: Najin and Fatu are the only living northern whites still producing eggs. “If they were to die today their eggs would go with them,” Vigne says. They’re also mother and daughter. So new eggs will have to be made from other rhinos — otherwise the northern whites will be too inbred to thrive.

The stem cell technique, which has nothing to do with cloning, has already been used in Japan to create live mice. Rhinos are much less studied than mice — but Jeanne Loring, director of the Center for Regenerative Medicine at the Scripps Research Institute, has already turned northern white rhino cell cultures into pluripotent stem cells. Her lab even turned those stem cells into nerve cells and beating heart cells in a petri dish, she says. So she’s confident she’ll be able to make sperm and eggs, too. “It’s just another cell type, so we don’t consider it to be an overwhelming challenge,” Loring says. “We do have to go about it with a lot of trial and error.”

Whatever technique the scientists use to create the northern white rhino embryos, the plan is to implant them into female southern whites kept in zoos. The two subspecies of rhino are so similar genetically that southern white mothers should be able to give birth to a northern white baby without too many problems, Ryder says. The first calf could be born in a few years, according to Stejskal. But “it’s difficult to say because there are so many steps,” he says. “There could be obstacles that we’re not aware of.”

Breeding a herd of northern whites and reintroducing them to Africa will be hard, and the project is risky. The technology might not even work — and then, millions of dollars might have gone to waste. But that’s okay, says Henry Greely, director of the Center for Law and the Biosciences at Stanford University, who’s not involved in the project. Pharmaceutical companies spend millions trying to develop new drugs, but often, they fail. “That happens a lot in science,” Greely says. Plus, the longer it takes to create a herd, the harder it’ll be to reintroduce it into its native habitat, says McCauley at UCSB. “Ecosystems change and the longer you wait, the more awkward it is to put a species back into place,” he says.

“That would be the most dramatic kind of statement.”

Still, the project is worth it, says Vigne at Ol Pejeta. It’s not just about reversing extinction, “it’s about preserving a genetic code which has evolved over millions of years,” he says. It’s about preserving the big, charismatic animals that enrich our planet. For Bennett, at Carleton University, the project could even work as “an effective poster child” that rallies the public about conservation and might encourage people to donate.

For many of the scientists involved in the project, it’s a matter of responsibility. Northern white rhinos are almost extinct because humans poached them to death, so it’s up to us to fix our mistakes. “The northern white rhino is emblematic of our times, about the impact of humans on wildlife,” Ryder says. “That would be the most dramatic kind of statement for what we could do in our time to reverse something that otherwise looks like a pretty sad commentary on our species.”