Finding them is another matter, when clandestine events can be announced with a fleeting Facebook post or text message and, in many cases, vanish after the music stops.
"You have a situation where folks are coming together and they are not applying for special permits. They're just posting fliers 24 hours in advance, and they're bringing hundreds of people in for different functions," Oakland Fire Chief Teresa Deloach Reed said at a news conference days after the Dec. 2 fire during an electronic music party at an illegally converted warehouse nicknamed "Ghost Ship."
"We do not have the resources to track those types of functions," she conceded.
After the blaze, officials in LA, Baltimore and other cities announced plans to aggressively pursue illegally converted warehouses and other jerry-rigged living spaces. The threat of a crackdown is unnerving musicians and artists who live in them, and routinely accept risks that can come with performing on unlicensed stages.
The Los Angeles case has similarities to Oakland, where a leased warehouse was converted into living space and an entertainment stage without proper permits or inspections.
"Lesser-known artists are happy to play nearly anywhere that will host them, because there are very few options," said Amanda Brown, co-owner of the Los Angeles record label 100% Silk, which lost two of its artists in the Oakland fire.
"These events are way more about community and shared experience than they are making money," she said in an email. "Most artists are very flexible and willing to deal with strange venues as long as there is a sound system and some enthusiasm for the music."
It's difficult to generalize about underground music and the places it's played, which sometimes hide in plain sight. Purple 33 has a web page.
It's like a jukebox, encompassing everything from punk to metal to electronic, a branch that itself is divided into dozens of splinters. Similarly, the events can range widely: a couple of dozen people in a garment factory, an after-hours gathering in a coffee shop, 150 electronic music fans in a clearing in a forest.
The scene is alternately inclusive, welcoming artists and fans of all demographics, and exclusive, since by definition it's hard to find if you don't know where to look. The list of Oakland victims speaks to the diversity it attracts: a teacher, a computer engineer, a filmmaker, musicians and artists, a lawyer.
Regulations vary, but generally a gathering of 100 people with live music on a stage would require one or more permits. Depending on its size, a fire marshal could make a spot check to ensure fire extinguishers are available and lighting is adequate, or officials might oversee the event.
In general, building inspectors in Los Angeles would reject applications for musical events in warehouses, since they are designed to hold goods, not parties.
A gritty location can be part of the allure, and Cassel and others see it as an escape from mainstream clubs that they see as unwelcoming, even hostile, to free spirits and nonconformists.
But a big part of it is economics. A maze of rules and the high costs that come with meeting them leave few alternatives for running events on a skimpy budget, they say.
"When you are charging five, six bucks and 50 people show up, it doesn't work to have a permit," said George Chen, who organized underground shows in the San Francisco Bay Area for over a decade.
Cassel acknowledges he ran Purple 33 without proper authorization, but says it's not because he didn't try.
The businessman-contractor-inventor who once sold skateboard parts claims he spent $70,000 on licensing and other fees to try to get the city to green-light his club, whose patrons he calls "a family."
But neighbors who feared wild parties blocked the project. He went underground.
"I wanted to do everything legal. It takes a lot of money," Cassel, 56, said in an interview.
"But the greatest issue isn't the money. They don't seem to want to guide you along. At the end of the day, they say no."
Despite Fire Department concerns, Cassel says the site was safe, with multiple exits, fire extinguishers and an emergency plan that could get everyone out in two minutes.
Fans note that legal venues have risks, too, pointing to a Rhode Island nightclub fire in 2003 that killed 100.
Given the numerous underground shows around LA, there appear to be relatively few reported problems.
The Los Angeles Fire Department dispatched 102 inspectors to check reports of overcrowding or illegal gatherings in 2016, but they do not document how many were in unpermitted locations.
The LAFD said it had no records of injury or damage at underground events.
Cassel hopes to reclaim his warehouse space one day, and he's partnering with underground organizers to change laws to make cities friendlier to what they do.
A crackdown will backfire, he predicted, and dangers could get worse.
"It's just going to go deeper underground."
Associated Press writer Paul Elias in San Francisco contributed to this report.