In a single day, the world’s population of northern white rhinos declined by 25 percent.

All it took was one last ragged exhalation from a 41-year-old rhino named Nola, one of four of the massive, two-horned animals remaining on Earth.

Now there are only three.

Nola was euthanized at the San Diego Zoo Safari Park on Sunday after an illness from a bacterial infection and age-related health issues worsened, the park said in a statement on its Facebook page.

“We’re absolutely devastated by this loss,” the park said, “but resolved to fight even harder to end extinction.”

The northern white rhino (Ceratotherium simum cottoni) is a subspecies of white rhinoceros that once ranged across parts of Uganda, Sudan, Central African Republic, the Democratic Republic of Congo and into Chad and Cameroon. They’re among the largest land animals on Earth, second only to elephants. They can run at speeds of more than 30 miles per hour and their horns, tough skin and sheer size mean they have no natural predators.

Unnatural predators — i.e., humans — are an entirely different matter.

Civil war, habitat loss and poaching for their valuable horns decimated the rare breed; between 1960 and 2015 the population fell from 2,000 to five.

This year, that number fell to three: In addition to Nola, a 31-year-old Czech rhino named Nabiré died after complications from a ruptured cyst in July.

The subspecies has been extinct in the wild since 2008.

Now just three aging members of the subspecies survive at Ol Pejeta Conservancy, a preserve in Kenya. All are too old or unhealthy to breed naturally — barring scientific intervention, when these last three die, their subspecies dies with them.

The northern white rhino “disappeared right in front of our eyes,” Kathleen Garrigan, a spokesperson for the African Wildlife Foundation, told National Geographic after Nabiré’s death, “and we didn’t realize it until it was too late.”

Though its numbers once tumbled as low as Nola’s subspecies, the southern white rhinoceros, a close relative, has fared far better. According to Save the Rhino, more than 20,000 of the creatures exist in the wild after being hunted almost to extinction a century ago.

In 1900, fewer than 20 of the rhinos existed on a single South African preserve. But good fortune and local governments favored the southern white rhino: A combination of legal protections, breeding efforts and regulated trophy hunting initiatives helped bring the subspecies back from the brink. Poaching persists — a record 1,215 white rhinos were killed in South Africa in 2015, according to the World Wildlife Fund. But by microchipping their prized horns and cracking down on the illegal ivory trade, officials hope to slash that number.

Northern white rhinos are unlikely to have such a comeback story. The three who remain live under constant surveillance in Kenya, surrounded by armed guards 24 hours a day. The lone male is too old to mate, and the two females are incapable of carrying a pregnancy, according to Live Science.

It would take a miracle to even produce another northern white rhino, let alone revive the subspecies. But scientists are hoping they can manufacture one.

Veterinarian Barbara Durrant of the San Diego Zoo’s Institute for Conservation Research told Live Science that she and other researchers are working to develop a rhino IVF procedure that would allow them to implant an embryo produced from preserved egg and sperm. But the number of preserved reproductive cells is tiny — they come from just 12 northern whites — and the procedure is still a long way from being ready to use them. Durrant said it’s only been done twice, and both times the embryo never developed beyond a cluster of just two cells.

The San Diego Zoo has set aside $2 million for an effort that would use southern white rhino mothers as surrogates to carry northern white rhino embryos, according to Scientific American.

If all else fails, southern white rhino females may also be artificially inseminated with preserved northern males’ sperm, preserving some of the genetic traits that make the northern subspecies unique. This step, though less than ideal, would at least add to the rhino population’s genetic diversity.

Meanwhile, staff at Ol Pejeta have been hurrying to harvest eggs from the last two living females.

“It’s kind of a race against time,” the preservation’s chief executive Richard Vigne told Live Science in June of this year when the global population of the subspecies stood at five animals. “Those remaining females could all die tomorrow. Once they’re gone, then the source of eggs disappears.”