FRESNO, Calif. >> In methamphetamine’s seedy underworld, traffickers are disguising the drug as a liquid to smuggle it into the United States from Mexico.

Dissolved in a solution, it’s sealed in tequila bottles or plastic detergent containers to fool border agents and traffic officers. Once deep in California’s Central Valley, a national distribution hub, meth cooks convert it into crystals — the most sought-after form on the street.

Tough policing has driven the highly toxic super-labs south of the border where meth is manufactured outside the sight of U.S. law enforcement, but the smaller conversion labs are popping up domestically in neighborhoods, such as one in Fresno where a house exploded two years ago.

People inside the home had sealed it tightly so the tale-tell fumes didn’t give them away.

“These guys, they don’t have Ph.D.s in chemistry,” said Sgt. Matt Alexander of the Fresno County Sheriff’s Office. “They’re focused on not getting caught.”

Investigators say it’s impossible to know how much liquid meth crosses the border, but agents in Central California say they have been seeing more of it in the past few years.

A California Highway Patrol officer in late 2012 pulled over a 20-year-old man on Interstate 5 who said he was headed to Oregon from Southern California and seemed nervous. The officer found 15 bottles in the trunk full of dissolved meth but labeled as Mexican tequila.

The man pleaded guilty to drug trafficking and received a federal prison sentence of 46 months.

Three men were indicted in late 2013 and await trial after a drug task force found 12 gallons of liquid meth in a Fresno house along with 42 pounds of the drug ready for sale, four guns and 5,000 rounds of ammunition.

Officers raided a Madera home earlier this year, finding a lab used to convert liquid meth into 176 pounds of crystals with a street value over $1 million. Nobody was arrested, but agents said the bust dealt a blow to the organization behind the lab.

Mike Prado, resident agent in charge of the U.S. Department of Homeland Security Investigation’s Fresno office, said law enforcement agencies are always on the lookout for creative ways cartels smuggle meth.

“We’ve become better at detecting certain things,” Prado said. “When they catch on to that, they modify their methods.”

The super-labs driven south to Mexico are notoriously toxic to people and the environment, but Prado said the small conversion labs in the Central Valley are more dangerous. His agents have found them in densely populated apartment buildings and foreclosed homes in quiet neighborhoods where children play on the street.

In the conversion process, cooks evaporate off the liquid and use highly combustible chemicals such as acetone to make crystals. The fumes are trapped inside. “A spark can turn this into a fireball,” Prado said.

That’s what happened in 2012, when a home in a middle-class area of Fresno was blown off its foundation. The blast shot the air conditioner into a neighbor’s yard; another neighbor had to replace a roof rippled by the concussion. Two men ran from the home, and investigators said a third was seriously injured.

Central California’s interstates and proximity to Mexico make it an attractive distribution hub for cartels, officials say.

John Donnelly, until recently in charge of the U.S. Drug Enforcement Administration’s Fresno office, said agents all over the country have tracked meth to California’s Central Valley. “We’re the source point for Seattle, Portland, Alaska and as far east as the Carolinas,” Donnelly said.

Not all the meth travelling north makes its way to Central California. Two men were arrested last month in San Bernardino when investigators found a conversion lab, 206 pounds of crystal meth and 250 gallons of the liquid capable of producing 1,250 pounds of crystals.

The seized drugs, which investigators suspect came from Mexico, were valued at $7.2 million.

Not all liquid meth makes it across the border. Last year, a 16-year-old from Mexico was stopped at the crossing near San Diego. He volunteered to take “a big sip” to convince inspectors the liquid he had was only apple juice, not meth. The teenager began screaming in pain and died within hours.

Eric L. Olson, a Latin America researcher at the Woodrow Wilson International Center in Washington D.C., said he witnessed agents seize liquid meth disguised in soda bottles during a 2012 tour of the border crossing at Laredo, Texas.

Liquid meth is just the latest innovation for transporting drugs for profit, he said. Smugglers have used tunnels, submarines, drones and once, Olson said, a 90-year-old farmer was used as a decoy.

“There’s no end to the creativity to getting the drug to market when there’s demand,” he said of the turn to liquid meth.