Sufferers of arachnophobia have a whole new family of spiders to fear - the Trogloraptor or cave robber, which is being described as a "fierce predator".
The spider, said to be "relatively huge" at 4cm across (1.6ins) with its legs extended, has "spectacular, elongate claws", experts said.
It was discovered by cave conservationists and scientists in America's Pacific north west and they believe many more of them may be lurking in the region.
The California Academy of Sciences said: "Their extraordinary, raptorial claws suggest that they are fierce, specialised predators, but their prey and attack behaviour remain unknown."
It adds: "The novel combination of evolutionary features in this spider, Trogloraptor, compelled them to recognise a new family."
The spider has been named cave robber because of its cave home and vicious looking claws. It hangs in rudimentary webs beneath cave ceilings, said the academy.
"It is a spider so evolutionarily special that it represents not only a new genus and species, but also a new family (Trogloraptoridae).
"Even for the species-rich insects and arachnids, to discover a new, previously unknown family is rare.
"The true distribution of Trogloraptor remains unknown: that such a relatively large, peculiar animal could elude discovery until 2012 suggests that more may be lurking in the forests and caves of western North America."
A team of "citizen scientists" from the Western Cave Conservancy and arachnologists from the California Academy of Sciences found the spiders in southwest Oregon.
Colleagues from San Diego State University found more of them in California's old-growth redwood forests.
Strong evidence suggests it is a close relative of goblin spiders (Oonopidae), but Trogloraptor possesses a mosaic of ancient, widespread features and evolutionary novelties.
A study of the new family and its evolutionary and conservation significance was published in the open access journal ZooKeys.
The forests in the coastal regions from California to the Canadian province of British Columbia are known for hosting unique and ancient flora and fauna, including tailed frogs, mountain beavers and coast redwoods.
"If such a large and bizarre spider could have gone undetected for so long, who knows what else may lurk undiscovered in this remarkable part of the world," the study said.