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.