Should you be lucky enough to find yourself on Hawaii’s Big Island, you are welcome to visit Mauna Loa, the largest volcano in the world.

Cats, however, are not — a point now being enforced by a new 5-mile-long fence constructed for the sole purpose of keeping felines away.

Mauna Loa’s lava-covered slopes make for some seriously forbidding landscape, but that hasn’t deterred cats, which have adapted to the Hawaiian islands just fine since arriving on explorers’ ships. So fine, in fact, that the little invasive predators are now a mortal threat to the endangered Hawaiian petrel, a seabird that breeds on Mauna Loa. Several thousand of the birds live in Hawaii, but only about 75 breeding pairs are in Hawaii Volcanoes National Park.

To protect the petrels, the National Park Service and other organizations spent nearly four years flying in people and materials to build the cat-proof barrier, a 6-foot-tall fence topped with a curved section that even the wiliest kitty is not supposed to be able to scale. It’s the longest anti-cat fence in the United States, and it encloses 600 acres of 8,000- to 10,000-foot-high terrain that petrels, also known as ‘u’au, now view as choice breeding territory.

Hawaiian petrels are seafaring birds that arrive at Mauna Loa to build nests in deep lava crevices in April. In June, females lay just one egg. The egg hatches in August, and the chick does not fly away until November.

That leaves seven months for feral cats to scale the slopes and feast on adult or baby petrels, which they have been caught on video doing.

The barrier is the latest example of a “conservation fence,” which is meant to protect wildlife from attackers (while also not angering cat-defenders too much). Hawaii is home to a handful, but the leaders in this method are Australia and New Zealand, where cats and other invasive predators have helped wipe out so many species that the governments have declared all-out war on them. One such fence in Australia, for example, is 26 miles long.

A previous version of this article incorrectly reported that there are about 75 breeding pairs of petrels on the Big Island. It has been corrected to reflect that that is the number of pairs in Hawaii Volcanoes National Park.

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