California had a growing problem with prison gangs, and decided the best way to deal with it was by locking up the leaders in a place so impenetrable and isolated, they'd be out of contact and out of business.

But things did not go according to plan. Lesley Stahl has the story of how a bunch of gangsters went to one of the most maximum-security prisons in the country, and turned it into their criminal headquarters.

Pelican Bay State Prison is a super-maximum penitentiary in Northern California. And yet with all the surveillance and isolation, gangs still run thriving criminal enterprises out on the streets – from within the fortress.

Lt. Steve Perez, a senior official at Pelican Bay, took 60 Minutes on a tour of the penitentiary. "These are the most creative, the most ingenious men, deeply committed to achieving their criminal goals," says Lt. Perez, who led the way to SHU, the security- housing unit, a special prison within the prison.

It's where gang leaders are housed in nearly solitary confinement. They're locked up in their cells 22-and-a-half hours a day. They are searched regularly for weapons, and their personal effects are X-rayed for contraband.

They get to exercise for about an hour a day in a small, dreary yard. Their only companion is a surveillance camera. And yet, Lt. Perez says, they manage to outwit the tight security to plot and scheme with one another.

"They'll use a drain right here, and they talk down through this drain," says Perez. "In fact, you can talk quite a ways down, because basically, when we designed the draining system, we connected a number of these individual exercise yards together, never thinking that this is how they would begin to communicate."

And there's little the prison can do about it, since with all the rain in Northern California, they can't plug the drainage system. "Now what we do is we pay attention to what they're saying," says Lt. Perez. "We listen as much as we can."

Perez showed other ways the inmates foil the security. He stopped at a cellblock for Brian Moore of the Aryan Brotherhood, a white supremacist prison gang. He's been in the SHU since 1999.

Moore, who is serving 18 years to life for second-degree murder, can't see his next-door neighbor, or anyone else on the block. And yet they communicate by what they call "fishing."

"You just take your little string line right here with a little weight on it, like that," says Moore, who makes a 40-feet-long line by tearing up his underwear or his bed sheet and braiding the threads together. He then fashions a weight out of soap. "And you just throw it out the door, and somebody else throws their other line out."

Just then, a written message came flying down from the upper tier. Moore demonstrated how he fishes, by flinging a line out, hooking it around the other line, and reeling it in. Once Moore ties the two lines together, the inmates easily send messages back and forth.

"If you know that they're fishing, and that they're sending messages like this, why do you let it go on?" Stahl asks Lt. Perez.

"We don't. The officers are required to search so many cells every day," says Perez. "The officer will come in. If he finds a fish line, he will take it at that time. The problem is that no sooner than I take it, he will go back to his bed sheet and he'll unravel the string again, the threads, and start the process all over."

With inmates who have nothing to lose, the authorities are left with nothing but frustration.

"Do I leave them without sheets? Do I take his T-shirts away? Do I leave him naked in his cell? How's that going to sound?" asks Lt. Perez.

Once they communicate with each other, the most effective way they get their messages to their foot soldiers on the outside is through the mail, which is one of their rights guaranteed by law.

Trained prison investigators like Devan Hawkes scrutinize every letter that goes in and out of the SHU, an average of 2,000 a day. One case involved a member of the Mexican Mafia.

"We reviewed over 1,000 letters that he sent out. And in those letters we found evidence of over 500 crimes that he committed," says Hawkes. "It involved drug trafficking, assaults on people, murder -- that he was either directing, or that was being reported back to him through the mail."

They use codes that have been so hard to decipher, they have been sent to FBI cryptologists in Washington. With nothing but time alone in his cell, Moore spent years learning an ancient Norse language that hasn't been used since 600 A.D.

Like other inmates, Moore sends out encoded messages embedded in intricate artwork. "I would mail it to a girlfriend, and then a girlfriend would mail it to a homeboy, or somebody else, somebody that I wanted to pass the message to," he says.

"And so what happens is that when this goes out, if you're not paying attention to what's happening, if you're not looking for the indicators of how they communicate, a beautiful piece of artwork becomes a message to have someone killed," says Lt. Perez.