Authorities called the burglary crew the Hole in the Ground Gang.
In the mid-1980s, the thieves tunneled under three L.A.-area banks, zipping through the underground in all-terrain vehicles.
They broke into two of the banks, making off with about $270,000, as well as the contents of 36 safe-deposit boxes.
The FBI and Los Angeles police "had never seen anything like this," retired FBI agent Bill Rehder and veteran journalist Gordon Dillow wrote in their book, "Where the Money Is: True Tales from the Bank Robbery Capital of the World."
Tunnel bank jobs are a rarity, involving not just engineering talent but hard work -- what lazy criminal wants to do all that digging?
The break-ins gave authorities nightmares of "the ground beneath us laced like a prairie dog village with holes and chambers and secret passages," the authors wrote, "of waking up one morning and finding 5 or 10 or 100 bank vaults simultaneously breached."
The Hole in the Ground Gang's first job was a First Interstate Bank at Spaulding Avenue and Sunset Boulevard.
In June 1986, bank officials opened for business on a Monday morning to find a 20-by-25-inch hole in the bottom of the vault, deposit boxes strewn all over the floor and about $172,000 in cash missing.
The contents of the boxes included a sketch of a young girl by Matisse.
Investigators discovered a 100-foot-long tunnel that led to a storm drain. The thieves had punched a hole in the drain, which they apparently covered with plywood and fresh mortar each time they left.
To excavate the tunnel, the gang -- working at night and on weekends -- moved more than 3,000 cubic feet of dirt, making approximately 1,500 wheelbarrow trips during the excavation.
The sewer water would be dammed up, then released each night to sweep the dirt through the drain, investigators believed.
Using a gas-powered generator for electric power, the intruders cut through the 18-inch concrete vault floor with saws and drills.
They left behind little evidence, aside from some equipment and "an enormous number" of empty Styrofoam coffee cups.
As the weeks went by, the FBI and the Los Angeles Police Department received numerous tips -- the villains were city workers, or Vietnam vets familiar with Viet Cong tunnels, or Viet Cong themselves, or a "gang of troglodytes, derelicts and winos who inhabited sewers."
Nothing checked out.
Then, a year later, tunnelers broke into a Bank of America vault at Pico and La Cienega boulevards, absconding with about $98,000.
There were signs the intruders had made a hasty exit after setting off an alarm. They left stacks of cash behind.
And some evidence was found. A check of area all-terrain-vehicle dealers found that four abandoned vehicles had been purchased by men described as "white males, early 30s, slim and muscular, dressed in construction-style clothes, speaking accent-less American English."
Authorities were certain it was the same gang. The buyer was listed as "Robert Spaulding" -- an apparent inside joke referring to Spaulding Avenue, one of the cross streets of the first victimized bank.
While the second burglary incident was fresh, authorities made a stunning discovery -- a third tunnel, which stopped near a vault at a Union Federal Savings and Loan on Wilshire Boulevard in Beverly Hills.
That vault hadn't been entered; perhaps the burglars decided not to stretch their luck.
Whatever the case, they were never caught and the statute of limitations on their crimes ran out in 1992.
Rehder is intrigued that the thieves were so inept inside the vaults, seemingly unaware of how to open the deposit boxes.
Aside from the $270,000 in cash they took, Rehder says the burglars may have gotten only "pennies on the dollar" for the contents of the 36 boxes, valued at $2.5 million. The other 78 boxes they hit were empty.
So the haul may have been less than $500,000. That's not so much if you deduct expenses, divide the loot among the half dozen or so gang members and factor in the many days and nights they spent at hard labor.
Perhaps profit wasn't their main motive.
Rehder speculated that perhaps they were construction workers "who had been members of an elite military unit at one time or had played football together. They were getting older. And maybe they wanted to experience a big thrill one last time."
Whatever, no tunnel bank jobs have been pulled in the L.A. area since -- except in fiction.
After reading about the real-life mystery, crime author Michael Connelly wrote his first novel, "Black Echo," about a similar heist.
Connelly recalls one reviewer complaining that the tunnel angle "was too far-fetched when, in fact, it was the only part of my novel that was based on fact."
After a 33-year career, Rehder retired from the FBI in 1999 and is part-owner of a security firm in Los Angeles.
His co-author, Dillow, a former columnist for the Orange County Register, lives in Arizona.
They both wonder about the whereabouts of that Matisse.