Remember the panic over "bath salts," the strange and terrifying new drug that supposedly turns people into face-eating zombies? The US Drug Enforcement Administration just announced a major crackdown on bath salts, "jewelry cleaner," "plant food," "spice," and other similar substances, seizing 550 kilograms of the stuff from around the world in just three days. The agency is calling it the largest synthetic drug bust ever.

Synthetic drugs cover a large and fast-growing class of new mind-altering compounds, which fall into two broad categories. The first covers synthetic marijuana, which sells under the street name "spice" or "K2," and is often labeled as incense. The second category is synthetic cathinones, which are drugs that mimic the effects of amphetamine and hallucinogens and produced a bout of horror stories, including the man who stabbed his neighbor's goat while wearing women's underwear, the man in Mississippi who skinned himself alive, and the woman who urinated on a $30 million dollar painting.

The news of the bust comes as the US State Department warned about the growing global threat of "new psychoactive substances," or NPS. The number of NPS reported rose more than 50 percent from the end of 2009 to mid-2012, according to a new report from the United Nations Office on Drugs and Crime.

Part of the problem is that the new drugs keep changing. If law enforcement identifies a new compound and classifies it as an illegal drug, underground chemists react by tweaking the composition slightly in order to produce a new, technically legal drug. There are now 251 different compounds identified as NPS, which exceeds the 234 substances regulated by international law.

"NPS are proliferating at an unprecedented rate and posing unforeseen public health challenges," the UN report says. "Since new harmful substances have been emerging with unfailing regularity on the drug scene, the international drug control system is now challenged by the speed and creativity of the NPS phenomenon."

The DEA and Congress have been working to catch up with the drug dealers. In 2012, Congress passed a law adding 26 new substances to the Controlled Substances Act. The DEA is relying on its emergency powers to temporarily add synthetic drugs to the list of controlled substances. The agency may also treat many drugs as controlled substances under the Controlled Substance Analogue Enforcement Act of 1986, if they're proven to be chemically similar to illegal drugs.

The DEA's crackdown reflected this global trend. Officials executed more than 150 arrest warrants and nearly 375 search warrants in 35 states and 49 cities in the US, Canada, Panama, Australia, and Barbados. The international cooperation is probably where the campaign got its name, "Project Synergy" (although it could double as a chemistry joke).

But even with today's big bust, we're likely to keep hearing about bath salts and other synthetics. "Shutting down businesses that traffic in these drugs and attacking their operations worldwide is a priority for DEA and our law enforcement partners," DEA administrator Michele Leonhart said in a statement. "These designer drugs are destructive, dangerous, and are destroying lives."