Until this Feb. 5, when fire erupted from a truck 2,100 feet below ground at the Waste Isolation Pilot Plant in southeastern New Mexico, the country's only permanent nuclear waste repository had a perfect 15-year-long safety record. It's hard to overstate how much this mattered: The deep salt-bed mine, called always by its acronym, only handles debris from nuclear bomb-making. Yet nuclear power proponents have long drawn optimism from WIPP's success. If a repository could succeed for nuclear-defense waste – a category less hot than spent reactor fuel – could we then find a place to inter the accumulating detritus of nuclear reactors? If WIPP could demonstrate safe geologic disposal, might other communities with the appropriate geology also welcome nuclear waste into their backyards?

Nuclear power could then make its comeback as a climate-friendly alternative to coal. That long hoped-for renaissance stalled out as more spent fuel rods crowded into inadequate pools at reactor sites – pools, as we saw at Fukushima Daiichi in 2011, that could burst into flames. WIPP offered hope, evidence, a living laboratory: Let 1,000 reactors bloom.

Had WIPP's problems ended with the truck fire, that vision might have had a chance. But nine days later, on Valentine's Day, alarms sounded above an air monitor, warning that radioactive particles were escaping from the mine. Workers quickly closed vents, redirecting air flow through particle-stripping filters. Still, alpha radiation from americium and plutonium, the fingerprint of nuclear-weapons waste, was detected a half-mile away.

No one thinks the truck fire triggered the radiation leak, but nearly four months later, no one knows for sure what actually did. There have been theories. "Theory after theory," New Mexico Environment Secretary Ryan Flynn complained at a Carlsbad town hall on May 8. One involved naturally occurring radon gas pressuring particles out of the mine; another suggested a fallen ceiling bolt had punctured a drum. "We need to be very careful what information we put out," Flynn said, at the same time chiding WIPP Recovery Manager Jim Blankenhorn, a U.S. Energy Department contractor, for his lack of transparency. "Evolving theories undermine confidence."

Blankenhorn maintained his reserve, and the next week described how workers dressed in protective gear and outfitted with extendable cameras had gathered clues: Images showing a container drum whose contents seem to have burst forth and frozen, like calcified curdled milk.  Bags of magnesium oxide, placed atop the drums to shield radiation, have spilled open. Dark stains on the drum suggest some sort of  "heat-producing event" occurred in nitrate salts contained in the drum. The damaged drum has been traced back to Los Alamos National Laboratories, one of several facilities that send Cold War-era defense waste to WIPP.

What caused the container to heat up? Investigators have a theory about that, too: Someone at Los Alamos packed the nitrate salts with the wrong kind of kitty litter, like Swheat Scoop instead of Jonny Cat.

No, really.

"In the '90s, when I was at Los Alamos, we actually did use Jonny Cat," says James Conca, an expert on nuclear waste disposal. "It has very nice chemical composition." Kitty litter absorbs any liquid that could usher radioactive material out of its container. But the wrong kind of kitty litter – a plant-based material as opposed to Jonny Cat's mineral clay – might have interacted with oxidizing nitrate salts and heated up, like "a slow-burning charcoal briquette," Conca says.

The Energy Department insists that the radiation release wasn't enough to imperil the public and had only minimal impact on workers. That's hardly reassuring: WIPP wasn't supposed to leak at all. Now that it has, it takes effort to believe that the next one won't be worse.

Nor does the accident at WIPP bode well for waste disposal from commercial nuclear power reactors, a problem so stymied by politics that the Obama administration recently stopped collecting disposal fees from utilities. The last attempt to develop a U.S. repository, at Yucca Mountain, Nevada, united all factions in opposition, not least because Yucca Mountain has a history of volcanism. WIPP, by contrast, has been dug into a 250-million-year-old salt bed laid down by the evaporation of the Permian Sea. "Salt is molecularly tight," Conca says. "It takes water a billion years to move an inch through salt." Yet the facility remains closed indefinitely. Some waste has been redirected to temporary storage in West Texas; more languishes on the surface where it originated.

Don Hancock of the nonprofit Southwest Research and Information Center in Carlsbad thinks the waste awaiting storage should stay where it is. WIPP, he says, "was a flawed site long before the February events. It's surrounded by more than 100 oil and gas wells." Others argue that closing WIPP would be a mistake, especially now.

"Everyone should be grateful that this waste drum vented after it was emplaced into WIPP," Per Peterson, a nuclear engineering professor at University of California, Berkeley, wrote in an email. "The worst thing that could result would be to leave remaining waste in limbo above ground, risking similar events." Peterson recommends clustering all suspect drums into WIPP's compromised sections and sealing them off forever. The salt bed will then isolate the radioactive waste, just as it did ancient bacteria, which have been found in the salt formation perfectly intact, DNA and all, after a quarter of a billion years.

But first, officials have to know what happened. Attempts to reproduce the organic kitty litter-nitrate salt reaction have failed, and new theories continue to emerge. Only one thing's sure: The promise of geologic disposal of nuclear waste has been dealt a huge blow. And the dimming hope of a nuclear waste solution won't brighten until WIPP's problems are fully understood – and maybe not even then.