Twenty-five years ago, Robert Cowie would climb atop the mountains of Oahu, Hawaii, and find one or two specimens of a brightly colored snail squirming around.

As a bioscience researcher at the University of Hawaii, Cowie would note the animal, one of many snail species that were identified as endangered on the island. But it’s been a long time since anyone has seen the snail, and researchers believe that’s probably because it’s gone extinct – along with many of its other sibling snails.

“I was probably one of the last people to see it alive,” Cowie said.

The Hawaiian archipelago is, geographically, the most isolated place on Earth. As a result, it’s home to a vast array of animals unique to the area.

But there’s another unique aspect of the island that’s making scientists concerned: Hawaii has more endangered species than any other U.S. state, and the rapid decline in species on the islands has given the area the unfortunate moniker of “the extinction capital of the world.”

Data on snail extinctions had led a team of international scientists to fear that the loss of biodiversity on the planet is far larger than previously predicted. Taking into account the millions of invertebrates often overlooked by researchers in the field, the researchers argue that the world has already lost 7 percent of its animal species, contributing to the long-theorized “sixth mass extinction” that is wreaking havoc on the planet’s biodiversity.

To date, only around 800 of Earth’s roughly 1.9 million species are officially recorded as extinct by the International Union for the Conservation of Nature. The low number has had skeptics question if there is really a crisis, but the methods used by the conservation group to declare a species as extinct are extremely rigorous and take a lot of time. To cover the enormous number of species in the world would take far more resources than would probably be possible, Cowie said.

To get a more complete idea of what’s really going on, Cowie and a team of researchers studied land snails throughout the world. The animal is the best invertebrate to study, because it leaves behind a trace that’s far more easy to observe: shells.

In a study published in the journal Conservation Biology, researchers determined that only 15 of the 325 species recognized in Hawaii can still be found alive and that the rate of extinction in the state has been as high as 14 percent per decade.

Snail species are a bit trickier to study globally, but with the help of a mighty network of snail experts across the world, scientists were able to look at 200 species in total. A tenth of the snails worldwide have gone extinct, they found. With that information, they were able to extrapolate the extinction rate to all other forms of invertebrates – such as the insects, spiders, arthropods and all the other spineless critters throughout the world.

Because invertebrates make up about 99 percent of all biodiversity on the planet, the study estimates somewhere around 130,000 species have gone extinct – way more than the 800 officially recognized.

“It’s a bit of a leap of faith, I admit,” Cowie told The Washington Post. “We just have to wave our arms around and say that’s the best we can do. ... Even if it’s not very accurate, you’re still going to come out with a huge number, it’s still a lot.”

“We’re not criticizing ICUN, they’re not set up to estimate the number of species,” Cowie added. “The point of the paper is to say, the vast, vast majority of invertebrates has not been assessed.”