On March 11, 2011, a 9.0-magnitude earthquake struck off Japan's east coast, creating a tsunami that knocked out the electrical power at the Fukushima Daiichi Power Plant—including the cooling system that prevented thousands of uranium-filled fuel rods in each reactor from melting down. Over the next few days, three of the plant’s six reactor buildings exploded, scattering nuclear fallout. Last summer, the Tokyo Electric Power Company, which operates the plant, admitted that at least 300 tons of radioactive water have leaked since then, some of it into the Pacific Ocean. More radioactive particles have escaped as steam and rained back down onto the ocean.

That contaminated water has been dubbed the “Fukushima plume,” and it has been slowly making its way eastward for the last three years. First, it mingled with water in the Kuroshio and Kurushio Extension currents off the Japanese Coast. Then it abandoned some of its radioactive cargo in the North Pacific gyre, also known as the “Great Pacific Garbage Patch.” This winter, as predicted, the plume was detected off the coast of Vancouver. It will reach the West Coast of the United States no later than this fall, and perhaps as soon as next month.

The plume has been covered only sporadically by the mainstream media, but it has become an obsession in certain corners of the blogosphere, even bridging the furthest poles of the political spectrum: Both the government conspiracy theorists on the far right, such as 9/11 truther Alex Jones, and the anti-nuclear environmentalists on the left have written extensively about the plume. As they see it, Fukushima's aftereffects pose a much greater risk to Americans than the government is letting on—cries of a "coverup" abound on Jones' Infowars site. Now, as the plume nears the West Coast, the fear is ratcheting up. On a message board on the green site Peak Oil, someone posted in January, “We all must come to the realization that swimming in the Pacific Ocean (let alone eating anything out of it) is a thing in the past.”

Some believe that radiation from Fukushima is already wreaking havoc in the U.S., linking it to starfish in California disintegrating into goo, polar bears in Alaska losing their fur, seals and walruses with “oozing sores,” and Pacific herring bleeding from their eyeballs. There are even claims that radiation is "already killing North Americans.” Scientists insist otherwise. “The polar bear losing fur, the three eyed-fish… There is no way that the levels we measure can explain these claims,” said Kai Vetter, a nuclear physicist at the University of California, Berkeley. But Vetter and others lack hard data to prove the Cassandras wrong—in part because the U.S. government has shown little interest in producing any. “In typical government terms, they’re saying, ‘There is nothing to be concerned about, just trust us,’” Vetter said. “They are not being very proactive in understanding the concerns of the public.”

Paranoia about the effects of nuclear radiation is as old as nuclear power itself. Tens of thousands of Americans equipped their homes with basement fallout shelters during the Cold War. After the Chernobyl disaster in the 1980s, Soviet nuclear experts coined the term “radiation phobia syndrome,” later shortened to “radiophobia,” to describe the “chronic state of stress” that posed “an even greater threat to health than exposure to radiation itself.” But Fukushima is something new: the first major nuclear disaster since Chernobyl, and therefore the first one of our digital age. The enduring, invisible boogeyman of radiation has met the limitless echo chamber of the Internet. Add the near-silence from the federal government, and it amounts to a perfect storm of paranoia that will only grow louder as the plume gets closer.