Last weekend, the streets outside a simple warehouse in San Francisco’s SoMa neighborhood were bustling with hundreds of people who were given an exclusive invitation to witness the Silicon Colosseum, an underground fight club featuring mechanical contenders.

Invitations were sent out online the night before to a select list of about 2,000 individuals. The event sold out quickly, with attendees paying $30 for the unique opportunity to see robots in combat against one another.

The invitation enthusiastically greeted, “Ladies and gentlemen, esteemed guests, and community members, investors in the future, and (of course) robots, with great excitement, I welcome you to the first showcase of cutting-edge robotic supremacy.”

As the event commenced, the excited crowd gathered around an enclosed octagon. The initial rounds were quite unusual, featuring volunteer participants engaged in “Taser knife fights,” using rubber blades mounted on stun guns to duel each other.

When the main attraction was ready, the “house robot,” a Booster T1 bot aptly named “Booster,” made its appearance as it was lowered into the ring. Standing at just under 4 feet tall and weighing 66 pounds, the boxing-glove-equipped bot received enthusiastic cheers as it demonstrated its shadow-boxing skills.

The evening’s first robot clash saw Booster face off against a taller adversary, K-Scale, a roughly 80-pound, headless machine, which Booster toppled multiple times. Later, Booster took on Gladiator, a bipedal, dog-like robot. Although Gladiator had the advantage of easily tipping Booster over, the humanoid maintained the crowd’s favor with its remarkable ability to recover and stand upright repeatedly.

Despite issues with some of the robots for Saturday’s card, the organizer behind the event, 20-year-old Bay Area transplant Verda Korzeniewski, told Nexstar’s KRON that attendees enjoyed the show.

“They loved it,” Korzeniewski said. “Somebody said it was just the right amount of weird. It spanned so many things: Robot fights, human fighting, quadrupeds versus humanoids.”

A robotics and software engineer by trade, Korzeniewski left behind his jobs in the robotics industry before focusing on her vision for the Silicon Colosseum.

“I was so addicted to robots that I gave up my entire life to do them,” she said.

Korzeniewski credited her fellow organizers for helping her pull off her second-ever robot fight night. (The inaugural Silicon Colosseum was held in July.) Shortly after the first show, she had also explained what it took to make the event happen.

“When I quit my job at a humanoid-robot company to start an underground humanoid-robot fight club, barely anyone believed in me or this idea,” she wrote on social media. “I had no money to buy robots and knew very few people who had the ability to get robots. Thankfully, I was able to find the best of the best, our rag-tag dream team. “The dream [is] still alive and ensouled in this city of madness and psychological warfare.”

Those interested in attending future robot battles are encouraged to join the Silicon Colosseum Partiful list. Korzeniewski said that she’s aiming to hold another robot fight in November; however, she didn’t reveal where the next event might take place.

“It’s an underground fight club,” she said. “You know I can’t tell you that.”